

I 











Operation of Dynamo 
Electric Machinery 


< 2 yxhJtAy^ 


\ 

By 

I.C.S. STAFF 


OPERATION OF DYNAMO-ELECTRIC MACHINERY 
Parts 1-2 



444 


Published by 

INTERNATIONAL TEXTBOOK COMPANY 


SCRANTON,PA. 

\<\ 2 . 2 . 


Ho* oKoD^”’ 


.Isi 


Operation of Dynamo-Electric Machinery, Parts 1 and 2: Copyright, 1922, 1906, by 
International Textbook Company. 


Copyright in Great Britain 


All rights reserved 


Printed in U. S. A. 



< i 
l ! t 


International Textbook Press 
Scranton, Pa. 


95130 







CONTENTS 


Note.—T his book is made up of separate parts, or sections, as indicated bv 
their titles, and the page numbers of each usually begin with 1. In this list of 
contents the titles of the parts are given in the order in which they appear in 
the book, and under each title is a full synopsis of the subjects treated. 


OPERATION OF DYNAMO-ELECTRIC MACHINERY 
(PART 1) 


Pages 


Selection . 1 

Installation . 2- 9 


Location; Foundations; Erection; Starting a generator; 
The belt. 


Operation of Direct-Current Generators. 10- 36 

Individual Parts of Machines—Their Defects and 
Remedies . ICC 19 

Brushes and Brush Holders. 10- 12 

The Commutator . 13- 15 

The Armature. 16- 17 

Field-Coal Defects. 18- 19 

Reasons for a Generator Failing to Generate. 20- 23 

Sparking at the Commutator. 24- 27 

Testing for Faults. 28- 34 


Open-circuited field coils; Short-circuited field coils; 

Grounds between winding and frame; Locating a 
ground; Defects in armature; Locating short circuit in 
armature coils. 

Repairs. 35- 36 

Field coils; Armatures; Balancing an armature. 

Operation of Direct-Current Motors. 37— 57 
















IV 


CONTENTS 


OPERATION OF DYNAMO-ELECTRIC MACHINERY 
(PART 2) 

Pages 

Direct-Current Generators in Combinations. 59- 68 

Generators in Series . 59 

Generators in Parallel. 60- 68 

Series generators in parallel; Shunt generators in paral¬ 
lel; Compound-wound generators in parallel. 

Alternating-Current Machinery. 69- 75 

Alternators in combination; Alternators in series; Alter¬ 
nators in parallel. 

Switchboard Appliances . 76- 95 

Field Rheostats . 76 

Switches . 77- 80 

Low-tension switches; High-tension switches. 

Fuses and Circuit-Breakers. 81- 83 

. Ground Detectors . 84- 88 

Protection From Lightning and Static Charges. 89- 92 

Measuring Instruments. 93- 95 

Voltmeters and ammeters; Wattmeters. 

Switchboards . 96- 99 

Direct-current switchboards ; Alternating-current switch¬ 
boards ; Personal safety from electrical shocks. 

Storage Batteries.100-107 

Primary and secondary cells ; Construction and charac¬ 
teristics of storage cells; Charging storage batteries. 















OPERATION OF DYNAMO- 
ELECTRIC MACHINERY 

Serial 839A (PART 1) Edition 2 


SELECTION AND INSTALLATION 


SELECTION 

1. A few general principles in regard to the selection of 
generators and motors apply to almost all cases. The con¬ 
struction of the machine should be first class in every 
respect. There should be evident solidity, each part being 
amply large, to insure durability, and as simple as possible; 
complicated parts are almost sure to cause trouble and 
expense. Machines in which careless workmanship, defect¬ 
ive material, or poor finish are evident should be avoided. 
If there is danger of mechanical injury from foreign sub¬ 
stances falling against the rotating parts, the machines 
should be provided with perforated doors or covers so 
arranged as to furnish the necessary mechanical protection 
and at the same time allow all possible ventilation. Electric 
machines for use in a damp atmosphere or one filled with 
dust or small flying particles of any kind should be entirely 
enclosed, dust- and moisture-proof, with suitable doors, or 
covers, for inspecting the working parts. This class includes 
motors for installation in mines, rolling mills, forge rooms, 
carbon works, cement works, etc. 

The machine selected should be of ample size for the 
work required and its construction, both mechanically and 
electrically, should be such as to require the least possible 


COPYRIQHTED BY INTERNATIONAL TEXTBOOK COMPANY. ALL R’OHTS RESERVED 




2 


OPERATION OF 


care and attention. The first cost of such a machine is not 
often of so great importance as is the cost of care and loss 
by breakdowns and repair bills. 

2. The losses occurring in electrical machinery are 
mostly converted into heat, which raises the temperature of 
the surrounding parts. In the purchase of such machinery, 
it is important that the temperature rise, as well as the 
sparking and overload conditions, be fully specified. It is 
not best to specify to manufacturers of electrical machinery 
many of the details of construction. The conditions to be 
met should be clearly stated and the specifications strictly 
adhered to. 

3. Finally, it is best always to deal with manufacturers 
of established reputation and to purchase machines so well 
standardized that duplicate repair parts can be quickly and 
easily obtained. Moreover, such concerns always keep in 
their employ competent engineers, who will give valuable 
advice as to an installation on which they are permitted 
to bid. 


INSTALLATION 

4. Location. —The location of a large generator or motor 
at a mine is generally determined by the location of the 
motive power and the work to be done. Aside from these 
considerations, however, the machine should be located in a 
clean, dry, cool place, protected from the dropping of water 
from steam and water pipes or from the roof. The location 
should afford, when practicable, a free circulation of air 
across the machine from windows or doors on opposite 
sides, but the air must be free from dust. 

A space surrounding the machine, especially around the 
commutator and brushes, should be clean and free from all 
obstructions. If the machine is controlled from a switch¬ 
board, the operator should be able to reach the board with¬ 
out going through a belt or over other obstructions. Dust 
from the street is injurious to the commutator, bearings, and 
general insulation of electrical machines, but dust from a 



DYNAMO-ELECTRIC MACHINERY, PART 1 3 


coal pile or any kind of grinding or turning machine is even 
more so, because it is often more adhesive, or sharper and 
more gritty; therefore, the dynamo should be protected 
from the dust incidental to coal handling, and no emery 
wheels, grinders, speed lathes, etc. should be allowed in 
the dynamo room. 

5. Foundations.—Every machine of 25 horsepower, or 
more, should be provided with a substantial foundation. In 
order to avoid communicating to the building the vibrations 
incidental to the running of the machine this foundation 
should, if possible, be independent of the floor and walls of 
the building in which it is installed. If several machines 
are to be installed in the same room, it will be best to 
have the whole floor space concreted and covered over 
with a layer of cement or a wood floor; but for a single 
machine it will be sufficient to make the foundation slightly 
larger than its floor area. In any case, stonework, solid 
brickwork, or concrete is the best foundation, but where these 
are impracticable, a substantial wooden frame construction 
can be used. When a concrete or brick foundation is used, 
it is customary to cap this with a hardwood frame, coated 
with a high-grade insulating compound of some sort; the 
layer of wood serves not only to insulate the metal frame of 
the machine from the ground, but it acts to cushion the 
blows and lessen the vibration due to the machine. 

Insurance underwriters have established certain rules, 
known as the National Code Rules, for installing elec¬ 
trical machinery, wires, etc. to which all such installations 
must conform before the buildings containing them are 
insurable against loss by fire. Installations in and around 
mines seldom need to conform to these rules, because most 
mining companies assume their own fire risks. One of the 
National Code Rules requires that the frames of generators 
and motors be thoroughly insulated from ground wherever 
feasible. This is the usual practice in installing generators 
at mine power stations, the wood base frame furnishing the 
necessary insulation. The motors used in the mines cannot. 


4 


OPERATION OF 


however, be so well insulated; and in any case where it is 
not feasible to thoroughly insulate the frame it should be 
permanently and effectively grounded, so that no one stand¬ 
ing on the ground and touching the frame may under any 
condition receive a shock. 

No rule in regard to the depth of the foundation can be 
given to cover all cases, as the subsoil is different in dif¬ 
ferent places. In one section, bed rock will be found a few 
feet below the surface, while in another section it will be 
necessary to drive piles to support the foundations for the 
heavier machines. Fig. 1 shows a style of foundation very 



Fig. 1 


much used; it is made of brick laid in mortar composed of 
one part of the best cement to two parts of good sharp sand. 
The sides of the excavation are filled in afterwards with a 
mixture of broken stone and cement, which is surfaced with 
a thin layer of pure cement. The masonry is built around 
the anchor bolts, which serve to hold the machine in place. 

If the machine is belt-driven, means should be provided 
for tightening or slacking the belt. This is usually accom¬ 
plished by mounting the machine on rails or on a subbase 
and moving it by means of a ratchet lever and screw, as 
shown in Fig. 2. The foundation should in every case be so 
disposed that the distance between the centers of the driving 
and driven shafts will allow one side of the belt to run 
looser than the other. This distance should be at least 
four times the diameter of the larger pulley. The loose 
side of the belt should be on top, the driving side below, 
as this will increase the arc of contact and the driving 
power of the belt. 





















DYNAMO-ELECTRIC MACHINERY, PART 1 5 


6. Erection. —Small machines are usually shipped com¬ 
plete and ready to run, so that it is only necessary to set 
them in place, line them up, and put on the pulley. Large 
machines cannot be shipped with safety in an assembled con¬ 
dition, and are, therefore, dismounted and the parts marked 
and packed in separate parcels. The assembling of the 
parts should not be undertaken by one wholly unfamiliar 



with such work, and even an expert must follow closely the 
blueprints and the marks on the parts. 


7 . Wherever machinery is installed, the apparatus neces¬ 
sary for handling the separate parts and for assembling the 
machine or taking it apart should always be available. If 
the installation is not of sufficient size to warrant the pur¬ 
chase of an overhead crane, a chain hoist may be made to 
serve the purpose. If the overhead timbers are not strong 









6 


OPERATION OF 


enough to support the heaviest weight, large trusses should 
be used. In any case be sure that the roof girders, cross¬ 
pieces, or trusses, as the case may be, as well as the hoist¬ 
ing apparatus, have ample strength for the work in hand. 

Definite instructions for setting up the machine usually 
accompany each instalment or at least will be supplied, on 
request, by the manufacturers. Unless the plant is a small 
one or the location so far away as to make it impractical, 
the manufacturers will usually supply an experienced man 
to superintend the installation. A few general suggestions 
may, however, be made; and, in order to be specific, let it be 
supposed that the six-pole, belt-driven, Westinghouse railway 
generator shown in Fig. 2 is to be assembled. 

8. The bedplate A and the lower half of the frame B are 
first worked into position, taking care in so doing not to 
disturb any part of the foundation. The lower field coils c , c 
are then put in place, taking particular care to assemble and 
connect according to markings on the coils. Hoist the arma¬ 
ture over its final position, but, before lowering it into place, 
see that the journals are wiped clean, that they are free 
from any bruises or scratches, and are covered with a thin 
film of oil. Slip the bearings over the ends of the shaft, 
following markings, if there are any, and lifting the oil rings 
so they will not be jammed or sprung. Wipe all dirt, sedi¬ 
ment, chippings, etc. out of the oil wells, lower the armature 
into place, and turn it a few times by hand to see that there 
is plenty of end play and that the oil rings turn properly. 
Fill the bearings with the best grade of thin lubricating oil 
but do not allow the oil to overflow, or oil throwing will 
result when the machine is started. 

By using a spirit level on the shaft, see that the machine 
is level, raising one side or the other, as is necessary, by 
placing split washers, or shims, of thin metal under the bed¬ 
plate around the anchor bolts. Put the remaining field coils 
in the upper half of the field frame and hoist it into position. 
All joints in the magnetic circuit, for example, between the 
two halves of the field frame and between the pole cores 




DYNAMO-ELECTRIC MACHINERY, PART 1 7 


and the frame, should be perfectly clean and coated with a 
thin layer of oil before being clamped together. These 
joints must fit perfectly and be thoroughly clamped. Now 
put on the pulley and line it up with the driving pulley to 
which it is to be belted. To do this, it may be necessary 
to turn the whole machine a trifle. Provision for doing 
this has been made by the slotted holes b , b in the corners 
of the bedplate. Lastly put on the brush-holder yoke, 
brush holders, and other fittings, and make all connections. 

9 . Too much caution cannot be used in handling such 
machinery, to see that it is not injured. A slight bruise or 
scratch on a journal or bearing or a bruised oil ring may 
result in a great deal of annoyance and possibly expense. 



Especial caution is needed in handling the field coils and the 
armature. It is imperative that these be not bruised or the 
insulation abraded in any way. The commutator is very 
sensitive to pressure or blows and should be shielded from 
them in every way possible. A very common way of hoist¬ 
ing an armature is shown in Fig. 3. The General Electric 
Company recommends for one line of generators an armature 
sling such as shown in Fig. 4; the rope makes two or more 
turns about the commutator, no two turns crossing each 
other. The pressure is thereby distributed all around the 
commutator. Knots should be tied in the sling to prevent 
the spreader from sliding down against the flange or end 
connections. This method may be convenient at times as 
it leaves the bearing free. It should, however, be avoided 

















8 


OPERATION OF 


unless the manufacturer of the machine being installed con¬ 
sents to its use. Lighter armatures may be handled as 
shown in Fig. 5. An armature must not be rolled or even 



laid on the floor where anything might possibly puncture the 
insulation or break a band wire, but must be supported by 



trusses under the shaft, or, if it must be laid on the floor, it 
must be protected by padding. 


10 . Starting a Generator.—Care must be taken to have 
the machine in perfect order mechanically before starting it. 
Turn the armature slowly by hand to see that it does not 
rub or bind at any point. Put on the belt, with the minimum 















































DYNAMO-ELECTRIC MACHINERY, PART 1 9 


distance between pulleys. See that all loose articles are 
removed from the machine. A good rule is never to allow 
a loose article of any kind to be placed on any portion of a 
generator. Start the machine up slowly and see that the oil 
rings rotate. When everything seems to be running smoothly 
and easily and without undue noise or vibration, gradually 
bring the machine up to speed and allow it to pick up its 
field. Tighten the belt until it runs steadily and without 
flopping and allow the machine to run several hours without 
load. If the windings have been exposed to dampness, it will 
be well to run at slow speed and a reduced voltage for a time, 
thus allowing the passage of sufficient current to dry out the 
moisture. After everything is in perfect order and the wind¬ 
ings are thoroughly dried out, the speed and the load may 
be gradually increased until the desired capacity is reached. 

11. Tlie Belt. —The generator belt should be endless; 
that is, it should have a cemented joint in preference to a 
laced joint, which may produce vibration and cause trouble. 
Any person who has approached a large rapidly moving 
leather belt may have noticed the peculiar sensation that 
accompanies slight electric shocks. Unless means are pro¬ 
vided to remove the static, or frictional, electricity that 
sometimes accumulates on large generator belts, the charge 
may become so heavy that it will jump through the air to 
the windings, puncturing the insulation and escaping to 
ground. The shocks from the frictional electricity on the 
belt may also sometimes be very disagreeable to the attend¬ 
ants. To prevent this accumulation of static charge, either 
the frame of the machine should be grounded or a metallic 
comb connected to earth should be so placed that the belt 
will run near the teeth and the charge will escape through 
the comb to earth. If the frame of the machine is insulated, 
as explained in Art. 5, a sufficient ground for the escape of 
the static charge but not sufficient to affect the frame insula¬ 
tion can be made by charring with a red-hot iron a fine line 
from a foundation bolt head along the wooden subbase to 
one of the bolts fastening the generator base. 


10 


OPERATION OF 


OPERATION OF DIRECT-CURRENT 
GENERATORS 

12 . Dynamo-electric machines and all devices connected 
with their operation or regulation should be kept scrupu¬ 
lously clean. No copper or carbon dust, dirt, grease, or oil 
should be allowed to remain on any part of the machine. If 
compressed air is available, a jet of air can be used at fre¬ 
quent intervals to blow all loose dust out of the commutators, 
armatures, field coils, etc. If this cannot be done, use a 
good hand bellows. Not only the machines themselves but 
all their surroundings should be kept perfectly clean and free 
from rubbish or litter. The appearance of the generator room, 
as well as that of the machines, indicates the alertness of 
the attendants and the probable attention given the whole 
plant. Continual watchfulness is necessary to discover any 
possible defect before it has developed sufficiently to cause 
serious trouble. It is well to follow a definite system of 
inspecting and caring for electrical machinery. Each part 
should be systematically examined, and cleaned or repaired, 
if necessary, at regular intervals. If this is done, there will 
be less chance of overlooking or forgetting anything, and 
expensive delays or repairs may be avoided. 


INDIVIDUAL PARTS OF MACHINES—THEIR 
DEFECTS AND REMEDIES 


BRUSHES AND BRUSH HOLDERS 

13 . On direct-current machines, the brushes and com¬ 
mutator require, perhaps, more attention than all the other 
parts of the machine. Brushes are of two kinds: radial and 
tangential. Radial brushes, Fig. 6 (#), point straight 
toward the center of the commutator. Tangential brushes, 




DYNAMO-ELECTRIC MACHINERY, PART 1 11 


Fig. 6 ( b ), frequently made of copper, are found, as a rule, 
only on low-voltage high-current machines. Radial brushes 
are nearly always made of carbon and are always used on 
machines designed to rotate in either direction. The 
brushes should be so placed that with a slight end play 
of the armature the whole commutator surface will be 
utilized. 

The pressure with which a brush should bear on the com¬ 
mutator depends on the material and condition of the commu¬ 
tator and the material of the brush itself. A copper brush 
does not, as a rule, require as much pressure as a carbon 
brush, and soft carbon will 
run with less pressure than 
hard carbon. Good practice 
is from li to 2 pounds per 
square inch. Pressures 
greater than 2 pounds per 
square inch are seldom nec¬ 
essary except where there is 
excessive vibration, as on 
railway motors. Increasing 
the pressure beyond what is 
necessary to maintain good 
contact only results in in¬ 
creasing the friction, with consequent heating and wear. 

14 . Carbon brushes are made in several grades of 
hardness, adapted to different conditions of working and 
different kinds of commutators. High-voltage machines 
usually require harder carbons than low-voltage machines. 
There are so many conditions affecting commutators that it 
is very difficult to specify the grade of carbon most suitable 
to a particular machine. The carbon must not be so hard as 
to scratch the commutator nor so soft as to cover it with 
smut. Harder carbons are generally used on electric-loco¬ 
motive and electric-car motors than for stationary work. 

Carbon brushes may be given a good bearing surface on 
the commutator by sliding a piece of fine sandpaper back and 





12 


OPERATION OF 


forth between the brush and the commutator, with the rough 
side next to the brush. Do not use emery paper on the 
brushes or the commutator, as emery is a conductor and 
may cause short circuits between adjacent commutator 
bars. Moreover, particles of emery sticking to the face 
of the brush, being more gritty than sand, will scratch the 
commutator. 

15 . Examine the brushes frequently when the machine 
is in operation to see that they have full bearing surface and 
that the surface is smooth and glossy. If the surface is raw, 
grayish in color, rough, and gritty, or if it is covered with 
particles or streaks of copper, something is wrong. Some¬ 
times conditions can be improved by changing the brush 
lead, that is, shifting the brushes, and often considerable 
relief can be had by boiling the brushes in vaseline. To do 
this, place the brushes in a vessel with sufficient melted 
vaseline to cover them and boil for about one hour, after 
which remove the brushes and wipe them dry. If there 
is time let them stand in an oven or other warm place for a 
few hours and wipe off all surplus grease before replacing 
them in the holders. 

16 . Metallic brushes are made of strips of copper, 
bundles of copper wires, or, more frequently, copper gauze 

folded into shape and 
stitched. Those made of 
strips or wires are very 
liable to have the edges 
or ends of the laminae 
fused together by spark¬ 
ing, forming hard points that cut the commutator. When¬ 
ever this occurs they should be taken out and the ends 
trimmed off. To get them to the proper level, so that they 
will rest evenly on the commutator at the proper angle, it 
is customary to use a filing jig, as shown in Fig. 7. This 
consists of a block of steel with a hole through it the size of 
the brush, and with one end beveled off to the proper angle 
and hardened. The brush is placed in the jig with the end 



Fig. 7 



DYNAMO-ELECTRIC MACHINERY, PART 1 13 


projecting a little from the beveled face, and clamped in 
position. The end of the brush may then be filed or ground 
down flush with the face of the jig, thus giving it the cor¬ 
rect bevel. 

Metallic brushes should not be allowed to become filled 
with oil or dirt; if they get in this condition, they may be 
readily cleaned with benzine or kerosene. 

17 . Brush Holders. —The moving parts of the brush 
holders should be as light as is consistent with strength, and 
there should be no stiffness or rigidity to prevent the brush 
from closely following any unevenness in the commutator. 
If carbon brushes are used, the brush, as it wears off, 
should move toward the center of the commutator and the 
pressure of the brush spring should remain practically con¬ 
stant until the brush is worn out. To prevent a tendency to 
chatter , or jump from the commutator, the brush holders 
should be set as near the commutator as possible. These 
points regarding brush holders are determined by the manu¬ 
facturer but will guide in selecting a machine. 


THE COMMUTATOR 

18 . The commutator is the most sensitive part of a 
machine, and its faults are liable to develop more quickly 
than those of any other part. When a commutator is in the 
best possible condition, it becomes a dark-chocolate color, is 
smooth, or glazed, to the touch, and causes the brushes, if of 
carbon, to emit a characteristic, squeaky noise when the 
machine is turning slowly. Oil should be used very spar¬ 
ingly, if at all, on a commutator; to lubricate it, put a film 
of vaseline on a canvas cloth, fold the cloth once, and let 
the commutator get only what oil goes through the pores. 
Too much oil or grease will cause arcing or flashing at the 
brushes and black rings will form around the commutator. 
These should be wiped off with a clean cloth. Never use 
waste to wipe the commutator or brushes, and the cloth used 
should be as free as possible from lint. 


444—2 



14 


OPERATION OF 


Some of the more common faults likely to develop in a 
commutator are roughness, eccentricity, and high or low 
bars. Any of these will cause sparking, flashing, or heating 
and unless attended to may soon render the machine inca¬ 
pable of further operation. 

19 . Roughness of the commutator may be due to over¬ 
loads, to improper setting of the brushes, to poor workman¬ 
ship or material, or to defective design. For occasional 
slight roughness, due to either of the first two causes, sand¬ 
paper may be used; but if the condition keeps recurring and 
seems to be due to either of the last two causes or to some 
other cause not readily ascertained, some more permanent 
remedy must be used. 

Before using sandpaper remove the brushes or fasten them 
back where they will be out of the way. Hold the sandpaper 
on the rotating commutator with a segment of wood having 
the same radius as the commutator. Use No. 2 sandpaper 
at first and finish with No. 0. For a final polish, reverse the 
paper and hold the smooth side next to the commutator for a 
moment. Blow all dust out of the machine as soon as the 
operation is completed. 

20 . Stoning. —Frequently, a commutator that appears 
very rough may be placed in a satisfactory condition by a 
process called stoning. A block of sandstone 4 inches 
square and 8 inches long can be placed in a wooden holder 
of convenient shape and size and one of the long surfaces 
made to fit the curvature of the commutator. Grinding a com¬ 
mutator with a stone made in this way is preferable to using 
sandpaper, for the stone will not dip into low places but will 
grind the high bars only. If the stone is coarse, it may be 
desirable to finish the commutator with fine sandpaper. The 
stone will not reduce the diameter of the commutator, or the 
radial wearing depth of the bars so much as a turning tool. 

21 . Eccentricity.—If a commutator is not properly 
baked during construction or is not screwed down after it is 
baked, it is liable to bulge out in the course of time under 


DYNAMO-ELECTRIC MACHINERY, PART 1 15 

the action of the heat due to its normal load and the action of 
centrifugal force, or it may develop loose bars. In the case 
of the bulging of one side, sandpaper will not do any good. 
The best thing to do with such a commutator is to take it 
off, bake it so as to loosen the insulation, tighten it up well, 
and turn it off in the lathe. For ordinary unevenness of 
surface of large commutators due to wear, it is customary 
to set up a tool post and a slide rest on the bedplate of the 
machine itself and turn off the commutator while in position. 
Commutator turning tools that may be readily attached to 
almost any large generator or motor are supplied by many 
leading manufacturers of electrical machinery. 

22. High or Low Bars. —If when a commutator is 
rotated slowly a sharp metallic click is heard as many times 
per revolution as there are brush holders, and a slight jump¬ 
ing of a brush is noticed every time the click is heard, there 
is probably one or more high bars. If it is a high bar and 
if it is tight in the commutator, the material in the bar is 
probably too hard; the bar may be dressed down with a file 
while the armature is standing still. A low bar may be due 
to soft material, to bad sparking caused by a defect in the 
armature winding, to a careless blow, or the bar may be 
loose. If due to any of the first three causes, the armature 
surface should be turned down in a lathe or with a commu¬ 
tator turning tool to the level of the low bar. If due to the 
second cause, the defect in the winding should also be found 
and removed. A loose bar, either high or low, will neces¬ 
sitate a thorough repair job. After turning a commutator 
always finish with No. 0 sandpaper as directed in Art. 19 . 
Inspect the surface closely to see that no burrs bridging 
across the mica have been left by the tool. 

23. The most serious condition is to have an armature 
or a commutator that is defective in design or that contains 
defective material or workmanship. If the design is a poor 
one, it may be very difficult or even impossible to keep the 
commutator in good condition. If the mica is too soft, it 
will pit out between the bars, leaving a trough to fill up with 


16 


OPERATION OF 


carbon dust and thus short-circuit the neighboring armature 
coils. If the mica bodies are too hard or too thick, the bars 
will wear in ruts and require frequent turning down. 


THE ARMATURE 

24. Heating. —An armature should run without exces¬ 
sive heating; if it heats so as to smoke or give off an odor, 
the machine should be shut down at once and the cause of 
the heating should be located and removed. The odor of 
overheated insulation is very peculiar and easily recognizable, 
especially after having once been experienced. The heating 
may be caused by damp insulation—a condition that, as a 
rule, is shown by steaming, but which can be determined by 
measuring the insulation resistance to the shaft with a volt¬ 
meter. If low resistance is indicated, the armature should be 
baked, either in an oven or by means of a current passed 
through it in series with a resistance which may consist of a 
number of lamps, known as a lamp bank , or as directed in 
Art. 10. The baking current should not exceed the full-load 
current of the machine. If, while the machine is at rest, a 
current for baking purposes be sent through the armature 
from an external source, be sure that the series-field, if the 
machine has one, is not included in the circuit, and that the 
shunt field is broken; for if either field is on, the machine may 
start up as a motor. 

25. Short Circuits. —If, instead of the whole armature 
running hot, the heat is confined to one or two coils, there 
is probably a short circuit either in a coil or between the two 
commutator bars to which the ends of the coil connect. If a 
short-circuited coil is run in a fully excited field, it will soon 
burn out. A short circuit of this kind can be readily detected 
by holding an iron nail or a pocket knife near the head of the 
armature while it is running in a field; any existing short 
circuits in the coils or commutator will cause the piece of 
metal to vibrate very perceptibly each time the defect passes 
underneath. If the trouble is confined to one or two coils it 
can frequently be located by stopping the machine after 



DYNAMO-ELECTRIC MACHINERY, PART 1 17 


running a few moments and feeling the armature all over for 
the hot coil. 

If one or more coil connections are reversed on one side 
of a generator armature, that side will generate less electro¬ 
motive force than the other, and hence, will receive current 
from the other side; that is, a current will flow through the 
armature coils that does not flow through the external circuit. 
This current is useless and heats the machine unnecessarily. 
It the same mistake is made in connecting a motor armature, 
the side having the reversed connections will generate less 
counter electromotive force than the other side and will there¬ 
fore receive more than its share of the current flowing through 
the motor, making this side overheat. 

26. A flying cross in an armature is a defect caused by 
a loose or broken wire with poor insulation; when the armature 
is standing still or even when it is rotating much below its 
standard speed, the wire may remain so nearly in place that 
the defect cannot be noticed; but when full speed is attained, 
centrifugal force throws the wire out of place and into contact 
with other wires or with the core or framework of the 
machine, causing sometimes severe sparking or flashing. 
Such a defect is often very hard to find; some of the tests 
given in Art. 25 may assist in locating it, or it may be necessary 
to give the whole armature winding a minute inspection. 

27. Overloaded Armatures. —One of the most common 
causes of general trouble and heating in an armature is over¬ 
load ; this may be due to ignorance or neglect or to an error 
in the instrument that measures the load. There is a great 
tendency on the part of owners to gradually increase the load 
on a machine until it may be doing much more than the work 
for which it was designed. By adding lamps one or two at a 
time it is an easy matter to unwittingly overload a generator. 
Or in the case of a motor, small devices may be added, one at a 
time, until an overload is the result. Ammeters sometimes 
get out of order, read incorrectly, or stick, and thus do not 
indicate the full load of the machine. 


18 


OPERATION OF 


FIELD-COIL DEFECTS 

28. Open Circuits.—Among field-coil defects are 

open circuits, short circuits, grounds, and wrong connections. 

An open circuit, or a break, occurring in the field circuit 
of a generator or a motor when the machine is idle, will usually 
be discovered on attempting to start up, before any further 
injury has resulted. If the break occurs while the machine is 
in service, the field magnetism will be lost, with results more 
or less disastrous, depending on the style of winding, the work 
the machine is doing, and whether it is operating alone or with 
other machines. For example, if the break occurs in the 
shunt field winding of a shunt- or a compound-wound generator, 
operating alone, the machine will merely cease to generate; 
if operating in parallel with other generators, as explained 
later, the other machines will be short-circuited through its 
armature with the possible burning out of some or all of the 
generator armatures on the circuit. A shunt motor will cease 
to generate counter electromotive force, and its armature will 
become a short circuit across the line and will be burned out 
unless the armature circuit is opened almost immediately. 
Application of the principles governing the generation of an 
electromotive force will enable one to determine the result of 
a break in the field circuit under conditions other than those 
given above. 

29. Short Circuits.—The effect of a short circuit in a 
field coil depends on the kind of machine and the method of 
field connection. If the defect occurs in a shunt field, there 
will be an increased field current, and but very little change 
in the speed of a motor or in the electromotive force of a 
generator. If a series-field is short-circuited, the effect in a 
generator is to reduce the electromotive force and in a motor to 
increase the speed; hence, if the electromotive force of a gener¬ 
ator becomes too low or the speed of a series- or a compound- 
wound generator becomes too high and the change cannot be 
otherwise accounted for, it is probable that the series-field has 
become short-circuited. 


DYNAMO-ELECTRIC MACHINERY, PART 1 19 


Short circuits may be caused by carelessness in winding or 
in handling, by defective insulation, or by moisture. By far 
the larger part of such defects are probably due to moisture 
absorbed by the insulating materials when the machines are 
idle for some time, especially if they are in a damp place. 
This moisture should be baked out either in an oven or by 
allowing a small current to flow through the coils for some 
time, increasing gradually to the normal current as the coils 
become dried. If very moist, the coils should be baked in 
an oven before sending a current through them. 

30. Grounds, or Connections, Between Windings 
and tlie Field Frame. —In circuits, neither side of which 
is permanently grounded, an accidental grounding of the 
windings will produce no further immediate injury to the 
machine, provided that the ground be removed at once; but 
if it be allowed to remain until a second one occurs the two 
will have the effect of a short circuit. On electric-railway 
circuits, however, where one terminal of the generator is per¬ 
manently grounded to the rails, a single ground on the 
windings will have the effect of a short circuit. 

31. Wrong Connections. —One or more field coils may 
be connected so that the current flows through them in the 
wrong direction, or the series and shunt coils of a compound- 
wound machine may be connected differentially, that is, so 
that they oppose each other in effect, when they were intended 
to be connected cumulatively, that is, so that they would 
assist each other in magnetizing the fields. It is a good plan, 
when connecting up a machine, to try the poles with a com¬ 
pass when the fields are excited, to see that the north and 
south poles alternate, and the series and shunt fields, if both 
are used, are connected in the right direction with respect to 
each other. 


20 


OPERATION OF 


REASONS FOR A GENERATOR FAILING 
TO GENERATE 

32. Among the causes for a generator failing to generate 
may be given, loss of residual magnetism; wrong connections 
of field or armature; open circuits or poor connections; short 
circuits; low speed; magnetic-circuit defects, that may consist 
of bad flaws, or blowholes, in the field casting or poor magnetic 
joints; wrong position of the brushes, etc. Some of these 
causes may result in a decreased voltage instead of a complete 
failure to generate. 

33. Loss of Residual Magnetism. —Of all the causes 
that may make a generator fail to generate, the loss of residual 
magnetism, or charge, is one of the most troublesome. As a 
rule, generators leaving the factory retain enough residual 
magnetism to start on, but there are several ways in which 
they can lose it. Some generators never lose their charge, 
while others are continually doing so. 

34. When a generator has lost its charge, the pole pieces 
have little or no attraction for a piece of soft iron. Series- 
generators seldom lose their charge so entirely that they fail 
to pick up a field on short circuit. When a compound-wound 
generator refuses to pick up a field with its shunt winding, it 
can often be made to pick up by disconnecting the shunt 
coils and short-circuiting the machine through a small fuse. 
Machines can in some cases be made to pick up a field by 
simply rocking the brushes back from their neutral position. 

If these expedients fail to produce the desired result, the 
fields must be recharged from an outside source. If the 
generator runs in multiple with other generators, it is only 
necessary to lift the brushes or disconnect one of the brush- 
holder cables on the dead machine and throw in the main-line 
switch, the same as if the machine were going into service 
with the others. The fields will then take a charge from the 
line and their polarity will be correct. If the generator does 
not run in multiple with another and there is a generator 
within wiring distance, disconnect the shunt field of the dead 
generator and connect it to the live circuit. If there are 


DYNAMO-ELECTRIC MACHINERY, PART 1 21 


absolutely no other means available for charging, several 
ordinary battery cells may be used. As a last resource, when 
all other available sources fail, connect the fields so as to 
obtain the least possible resistance, put them in series with 
the armature through a small fuse, and speed the armature 
considerably above the normal rate. Very often a generator, 
instead of losing its residual magnetism, will acquire one of a 
reversed polarity, due, perhaps, to the same causes exercised 
to a greater degree. In this case, the generator will build up 
with the polarity of the brushes reversed. In some cases this 
would do no harm, but in most cases it is essential that the 
brush polarity be always the same and if the generator begins 
to build up wrongly it is best to stop it at once and ascertain 
the cause. If it is found that the residual magnetism is 
reversed, an external electromotive force should be applied, 
as before indicated, to restore the fields to their proper 
direction. 

35. Wrong Connection of Field or Armature. —In 

the process of building up the field of a generator, it is essential 
that the very slight electromotive force due to the armature 
conductors cutting the residual magnetic field, shall send cur¬ 
rent around the field coils in such a direction as to add to the 
residual magnetism. If the reverse were true, all the mag¬ 
netism would be killed and the generator would fail to gen¬ 
erate. It follows, then, that if, after a generator has been 
left charged in one direction, its field or armature leads are 
reversed, the machine will not pick up; and, if it is run long 
with these wrong connections, the residual magnetism will be 
completely lost and the machine will fail to pick up, even 
when the connections are made right again, until the fields 
have been recharged. 

36. Again, one or more field coils may be incorrectly 
put on, or connected so that they oppose one another. On a 
compound-wound generator, the reversal of a shunt-field coil 
will generally keep the generator from picking up on open 
circuit, unless the generator has more than four coils; the 
more coils it has, the less effect will the reversal of a single 


22 


OPERATION OF 


coil have. The reversal of a series-coil is not felt until an 
attempt is made to load the machine; the voltage will not 
come up to where it should for a given load, and the brushes 
are apt to spark on account of the weakened field. 

37. Open Circuits or Poor Connections. —A shunt or 
compound-wound generator will not pick up if the shunt- 
field circuit is open; the open circuit may be in the field itself, 
in the field rheostat, or in some of the wires or connections 
in the circuit. A careful inspection will generally disclose 
any fault that may exist in a wire or connection. To find 
out if the rheostat is at fault, short-circuit it with a piece of 
copper wire; if the machine generates with the rheostat cut 
out, the fault is in the rheostat. A field circujt is some¬ 
times held open by a defective field switch that is apparently 
all right; repeated burning may have oxidized the tip of 
the switch blade and formed on it a non-conducting blister, 
which prevents the jaws of the switch from coming into 
electrical contact with the blades. Another trivial but com¬ 
mon cause of open circuits is the blowing of fuses. 

An open circuit in an armature will interfere with the 
proper generation of electromotive force, but such a fault, as 
a rule, announces its own occurrence and location in a very 
emphatic manner. There will be severe sparking and the 
commutator bars to which the open coil is connected will be 
badly burned in a short time. 

Before attributing the failure to generate to any of the 
foregoing open-circuit causes, see that the brushes are on 
the commutator, the field switch closed, and the greater 
part of the field rheostat cut out. The electromotive force 
generated when a machine is first started is very small, 
because the residual magnetism is weak. It may not require 
a complete open circuit in a field to prevent a machine 
picking up. A bad contact that might not interfere with the 
working of the machine when it is up to full voltage may be 
sufficient to prevent its picking up when first started. 
A loose shunt wire in a binding post, or a dirty com- 
,mutator may introduce sufficient resistance to prevent the 


DYNAMO-ELECTRIC MACHINERY, PART 1 23 


machine from operating. Trouble is very often experienced 
in making machines with carbon brushes pick up, especially if 
the brushes or commutator are at all greasy. If such is the 
case, clean the commutator thoroughly, wipe the ends of the 
brushes with benzine, and see that they make a good contact 
with the commutator surface. 

38. Short Circuits. —A short circuit occurring on the 
main line of a shunt generator while the machine is running 
will cause it to lose its field; therefore, the machine will not 
pick up if its line is short-circuited. A short circuit on the 
line of a series-wound or a compound-wound generator increases 
its ability to pick up, because the fault is in series with the 
series-coils and a large current passes through them. A 
series-generator cannot pick up with its external circuit open, 
because no current can flow through its field coils. Either 
a series or a shunt generator may not pick up if its field is 
short-circuited. A compound-wound generator may not pick 
up on open circuit if the shunt field is short-circuited; if the 
series coils only are short-circuited, the machine will pick up 
with the main circuit open, but will not hold its voltage when 
the current begins to flow. In some cases, a shunt generator 
will not pick up on full load, as this realizes too nearly the 
condition of a short circuit; so that to be on the safe side it is 
best to let the machine build up its field before closing the 
line switch. 

Short circuits within the generator itself generally give rise 
to indications that point out the location and nature of the 
fault. In any event, the first thing to find out is whether the 
fault is in the generator or out on the line; if the machine 
picks up its field when the line switch is opened, but fails to 
do so when it is closed, the trouble is on the line. 

39. Low Speed.— A generator will not pick up its field 
when running below a certain speed, but with the field once 
established, the machine will hold it at a much lower speed 
than that required to pick it up. The speed at which a series- 
generator will pick up depends on the resistance of the external 
circuit. 


24 


OPERATION OF 


40. Other Causes. —Defects in the magnetic circuit 
appear, if at all, when the machine is first assembled. Gen¬ 
erators with defective field castings are, of course, not allowed 
to leave the factories of reputable makers. Defective magnetic 
joints may be due to carelessness in assembling. The cor¬ 
rect position of the brushes, as found by the factory test, 
is usually marked in a conspicuous place on the generator 
frame near the brush-shifting device. In any case, this posi¬ 
tion should easily appear after a few trials, even if the mark 
cannot be found. 


SPARKING AT THE COMMUTATOR 

41. Probably the most troublesome and annoying feature 
in the operation of direct-current generators and motors is 
sparking at the commutator. The cause is not always 
apparent, but may usually be found among the following: 
Too much load; brushes improperly set; commutator rough 
or eccentric; high or low bars; sprung armature shaft; brushes 
making poor contact; dirty brushes or commutator; too high 
speed; low bearings; worn commutator; short-circuited or 
reversed armature coil; open-circuited armature; vibration; 
belt slipping; weak field; grounds. 

42. An overloaded armature heats all over. The 
sparking may be lessened but not stopped by shifting the 
brushes ahead on a generator and back on a motor. If 
the machine is a motor, the speed will be low; if a generator, 
the voltage will be below the normal amount, unless the 
machine is heavily overcompounded. 

Brushes may be improperly set in either of two ways: 
they may be the right distance apart but too far one way or 
the other as a whole; this can, of course, be remedied by 
shifting the rocker-arm back and forth until the neutral point 
is found. The brushes may, as a whole, be central on the 
commutator, but the spacing between adjacent holder studs 
be wrong. Count the commutator bars between adjacent 
sets of brush holders and adjust the spacing until the number 
of bars between each pair is the same. 



DYNAMO-ELECTRIC MACHINERY, PART 1 25 


43. Remedies for a rough or eccentric commu¬ 
tator were given in Arts. 19, 20, and 21. A sprung 
shaft has the same effect as an eccentric commutator; 
either will cause the brushes to jump from the commu¬ 
tator and sparking will result. Before turning the com¬ 
mutator for eccentricity be sure that the trouble is not due 
to a sprung shaft. 

A high or low bar in a rotating commutator causes the 
brush to jump from the commutator, and this gives rise to 
sparking. 

A sprung armature shaft causes the commutator to 
wabble, producing very much the same symptoms as an 
eccentric commutator. 

44. The brushes may make poor contact due, to a 
brush being stuck in a holder so that the spring does not 
force it down on to the commutator; to the temper being 
out of the spring; to the pressure of the spring not being 
brought to bear directly on the brush; to the brush not 
fitting the commutator surface, etc. 

Dirty brushes or commutator may cause the brushes 
to make poor contact. Some carbon brushes contain paraffin 
placed in them for lubricating purposes. When the brushes 
are hot, the paraffin may run out too rapidly and cover the 
commutator with a greasy smut, which insulates it in spots. 
Copper brushes sometimes get clogged with oil, dust, and 
bits of lint or waste. Dirty commutators are usually the 
result of using too soft brushes, or too much oil, or frayed 
cloths or waste in cleaning. 

45. Too high speed is apt to make a machine spark, 
because it affects the correct position of the brushes. 

Worn bearings sometimes throw the armature far enough 
out of the center to distort the field and cause sparking. 

A badly worn commutator, even if otherwise in good 
condition, seems inclined to spark in spite of everything 
that can be done. It may be because, as the bars wear 
down radially, they also become thinner and the brushes 
then span too many bars, in which case a thinner brush may 


26 


OPERATION OF 


give relief; or it may be because the error in the angle of the 
holder increases with the distance from the commutator. 

46. Either a short-circuited or a reversed armature coil 
will cause a local current that will increase the power required 
to run either a generator or a motor, even without any load. 
A motor will run with a jerky motion, especially noticeable 
at low speeds, and a generator will cause the needle of the 
voltmeter connected to its terminals to fluctuate. In either 
case, unless the cross that causes the trouble is removed, the 
coil will burn out. 

By an open-circuited armature is meant a break in one 
of the armature wires or its connections. Excessive current 
may bum off one of the wires or a bruise of some kind may 
nick a wire so that the normal load, or perhaps less, bums it 
off. A commutator may become loose and break off one or 
more leads. Sometimes, on account of excessive heating, the 
armature throws solder and all the commutator connections 
become impaired; in such a case, while there may be no actual 
open circuits, there are poor contacts that result in making 
the commutator rough and black. 

47. Vibration of a generator or a motor will cause con¬ 
stant sparking, even at very light loads. The vibration may 
be due to a poor foundation or to a poorly balanced armature; 
the remedy is to place the machine on a firmer foundation or 
to properly balance the armature. 

A slipping belt will sometimes cause intermittent sparking, 
because it subjects the machine to unusual variations in speed. 

48. Causes for weak fields have already been mentioned; 
viz., poor joints, either magnetic or electric, wrong connec¬ 
tions, short, circuits in series-fields, etc. A weak field mag¬ 
netism is easily distorted by armature reaction until it may 
become impossible to shift the brushes to a point of sparkless 
commutation. 

As in the case of field coils, a single ground on the armature 
windings of a railway generator, or any machine working on 
a permanently grounded circuit, will have the effect of a 
short circuit and will cause sparking and heating, as described 


DYNAMO-ELECTRIC MACHINERY, PART 1 27 


in Art. 30. On completely insulated circuits, two grounds 
on the generator armature windings will cause a short circuit 
with the same effects. 

49. The causes of sparking thus far mentioned are such 
as may be due to improper treatment or abuse after a machine 
has left the factory, and not necessarily the result of faulty 
design or construction. It sometimes happens, however, that 
notwithstanding a generator or motor receives only the best 
of care, it persists in sparking badly at full load or even less. 
This may be due to poor design, mechanically or electrically, 
something for which the attendant is not responsible, except¬ 
ing possibly as the machine may be one of his selection. 

50. A moderate amount of sparking at the commutator 
is not objectionable, but, if it becomes sufficient in amount or 
in duration to blacken or roughen the commutator bars, the 
cause should be located and removed if possible. Numerous 
small white sparks, evenly distributed along the edge of the 
brush and producing no distinguishable noise, usually work 
little injury. Larger sparks, appearing at irregular intervals 
along the edge of the brush, usually with a greenish hue and 
accompanied by a hissing sound, are more serious. Such 
sparks usually cling tenaciously to one point on the brush 
edge, and they are due to small particles of copper, torn loose 
from the commutator by excessive local heat and which cling 
to the brush surface. On stopping the machine after running 
a few hours with this kind of sparking, a furrow, or strip, will 
be found cut into the commutator all around the circum¬ 
ference under the spot where the spark appeared. Sparks 
due to incorrect position of the brushes, when load is changed, 
produce a vicious snapping sound, easily distinguished after 
having once been heard. A well-designed, modem, direct- 
current generator or motor, with the brushes in one position, 
should be sparkless from no load to full load and possibly to 
25 per cent, overload. There should be no injurious sparking 
at 50 per cent, overload and many manufacturers guarantee 
their machines to stand even 100 per cent, overload, momen¬ 
tarily, without injury. 


28 


OPERATION OF 


TESTING FOR FAULTS 


51. Many of the defects that are liable to develop in 
dynamo-electric machines are apparent from a mere inspec¬ 
tion. Other defects, such as short-circuited or open-circuited 
field coils or armature coils, must be located by making tests. 
For tests of this kind, the Weston or similar instruments are 
most convenient if they have the proper range for the work 
in hand. For measuring resistances, the drop-of-potential 
method is generally most easily applied. This method con¬ 
sists in sending a known current through the resistance and 
measuring the drop of potential between the terminals of the 
resistance from which the amount of resistance is calculated. 

For measuring a very low resist¬ 
ance as, for example, that of an 
armature coil, the voltmeter must 
be capable of reading low, say to 
thousandths of a volt. A milli- 
voltmeter will be best suited to 
this work. 


rdlllhr 




& 


VM 


Fig. 8 


52. The drop-of-potential 
method of measuring a resistance 
may be better understood by reference to Fig. 8, where 
it is desired to measure the resistance of coil R. An 
ammeter A is connected so that it will measure the current 
forced by a battery B , or any other source of electromotive 
force, through the coil; and a voltmeter VM, connected to the 
terminals t, measures the electric pressure across the coil, 
or the drop of potential in the coil. From Ohm’s law, current 


electromotive force 


or resistance 


electromotive force 


resistance current 

For example, if the ammeter indicates 1.5 amperes and the 
voltmeter 9 volts, the resistance equals 9 -f- 1.5 = 6 ohms. 


53. Open-Circuited Field Coils. —If a generator fails 
to pick up, and a voltmeter connected across the brushes 
shows a small deflection when the machine is running at 
full speed, the failure cannot be due to loss of residual 
















DYNAMO-ELECTRIC MACHINERY, PART 1 29 


magnetism. A careful examination will reveal any defective 
or loose connections between the coils. Quite frequently, the 
wire becomes broken at the point where the leads leave the 
spool, while the insulation remains intact, so that the break 
does not show. This may be detected by bending the leads to 
and fro. 

If the break, however, is inside the winding of one of the 
coils, it can be detected only by testing each coil separately 
to see whether its circuit is complete. To do this, connect 
the field directly across the circuit of another generator, if one 
is available, as in Fig. 9, where the field terminals are con¬ 
nected at a, e to wires coming from another machine in 
operation. If the field coils 
1, 2, 3, 4 were all perfect, a 
current would flow through 
them; but if one of them 
has a break in it, as at B, 
no current can flow. To 
locate the defective coil, 
the terminals of a volt¬ 
meter are touched to the 
terminals of the different 
coils until the defective one 
is indicated by a deflection 
of the voltmeter needle. 

The needle will in this case 
indicate drop of potential, 
terminals a, b , of coil 1, there will be no deflection of the needle 
because no current is flowing through coil 1, hence there is no 
drop of potential in the coil. When the voltmeter terminals 
are touched to terminals b,c of the defective coil, as indicated 
by dotted lines, it is connected through coil 1 to the posi¬ 
tive side of the circuit and through coils 3 and 4 to the 
negative side; hence, it will measure the full pressure of 
the circuit connected to a, e provided the other coils are 
perfect. 

If a generator circuit is not available for making the test 
illustrated in Fig. 9, a common battery and a bell in series, 



When the terminals are touched to 








30 


OPERATION OF 


or a magneto-electric bell such as ordinarily used for tele¬ 
phone signaling (called magneto for short), may be substituted 
for the voltmeter. It is evident that if connections are made 
at the terminals a , b , of coil 1 , or those of any other perfect 
coil, the bell will ring, but if made at b, c , or at the terminals 
of any other coil containing a break, there will be no ring. 

54. Short-Circuited Field Coil. —If the windings of a 
field coil become short-circuited, either by its wires coming 
in contact with each other or by the insulation becoming 
carbonized, the defective coil will show a much lower resist¬ 
ance than it should. The drop of potential across each of 
the various field coils should be about the same, so that, if 



one coil shows a much lower drop than the others, it indi¬ 
cates a short circuit of some kind. The short-circuited, coil 
will usually run cooler and all the others warmer than normal. 

55. Grounds Between Winding and Frame. —After 
a machine has thoroughly warmed up for the first time after 
being installed, and at frequent intervals thereafter, it should 
be tested for grounds. This may best be done with a good 
high-resistance voltmeter, as follows: While the machine is 
running, connect one terminal of the voltmeter to one ter¬ 
minal of the generator and the other terminal of the voltmeter 
to the frame of the machine, as shown in Fig. 10, where T 















DYNAMO-ELECTRIC MACHINERY, PART 1 31 


and Ti are the terminals of the generator and Y and Yi two posi¬ 
tions of the voltmeter, connected as described above. 

If in either position the voltmeter is deflected, it indicates 
that the field winding is grounded; the greater the deflection, 
the nearer the ground to the other terminal; that is, a large 
deflection at Y shows that the machine is grounded near 
the terminal 7\. If the needle shows a deflection in both 
positions, but seems to vibrate or tremble, the armature or 
commutator is probably grounded. If in either case the 
deflection does not amount to more than about one-twentieth 
the total electromotive force of the machine, the ground is 
not serious; but if the deflection is much more than this, the 
windings should be examined separately, the ground located, 
and, if possible, removed. Before making this ground test 
on a railway or other permanently grounded generator, the 
grounded terminal should be disconnected from the circuit. 


-Fig. 11 illustrates a method 
C, _ 


56. Locating a Ground, 
of testing to locate a 
ground. The machine is 
shut down and the electric 
circuit broken into as many 
distinct portions as possible; 
that is, each field coil is dis¬ 
connected from its neighbors 
and the generator termi¬ 
nals T, Ti are disconnected 
from the external circuit. 

C, Ci are terminals of a live 
circuit of about the same 
difference of potential as the 
normal voltage of the defective generator when running. One 
terminal C of the live circuit is connected to some bright 
surface on the frame (a bolt head in this case) where good 
contact can be had, and the other to a voltmeter Y of suffi¬ 
cient capacity to measure the full electromotive force of 
circuit C C\. The other voltmeter terminal is connected to 
successive field terminals t } etc. and if need be to the 




P“k 

d 




Fig. 11 






















32 


OPERATION OF 


machine terminals T , T„ or to the commutator. In each case 
little or no deflection will be shown until connection is made 
to the defective portion of the circuit. In the figure, if the 
coil with terminal /, were grounded, the voltmeter would 
show a deflection. If the ground were complete, that is, a 
dead grcnmd, the deflection would show the full voltage of the 
circuit C C x . 


57. Defects in tlie Armature.—Faults in the armature 

may best be located by 
what is known as the 
bar-to-bar test, con¬ 
nections for which are 
shown in Fig. 12. A 
current from an exter¬ 
nal circuit E is led 
through the armature 
by way of contacts A B, 
which may be clamped 
to the commutator. A 
variable resistance, 
represented by the lamp 
bank LB should be 
used to regulate the 
strength of this current. 
A millivoltmeter G is 
connected, through the 
commutator bars 1,2,3, 
etc., successively, to 
the individual coils 
N, W, K, S, etc., by 
means of a contact 
maker, or crab, C, which is provided with two properly 
spaced contact pieces. Suppose, in this case, that the 
generator has three defects, which are as follows: (1) There 
is a break in coil T, which prevents any current flowing 
through the bottom coils between the contacts A , B, but all 
the current passes through the top coils; (2) there is a short 




























DYNAMO-ELECTRIC MACHINERY, PART 1 33 


circuit in coil N ; (3) the commutator leads of coils S, K, W 
are mixed. All these defects are indicated in the figure. 

58. The test is carried out as follows: Adjust the lamp 
bank until the voltmeter gives a good readable deflection 
when C is in contact with what are supposed to be good coils. 
The amount of current required in the main circuit will 
depend on the resistance of the armature under test. If the 
armature is of high resistance, a comparatively small current 
will give sufficient drop between the bars; if of low resistance, 
a large current will be necessary. With the contact maker C, 
the operator runs over several bars to obtain what is called 
the standard deflection with which to compare all the other 
deflections. The damaged part will often show a wide differ¬ 
ence in deflection from the good coils. The deflection of the 
voltmeter will depend on the difference of potential between 
the bars. If everything is all right, the difference of poten¬ 
tial between each pair of consecutive bars will be practically 
the same. 

No deflection will be obtained on the lower side, except 
when bars 15 and 16 are bridged. There will then be a 
violent throw of the needle, because the voltmeter will be 
connected to A and B through the intervening coils. The 
break is thus located in coil T. As a temporary remedy for 
this, bars 15 and 16 may be connected by a jumper or piece 
of short wire. The defective coil T should, however, be 
repaired as soon as possible. 

When the contact rests on bars 3,4, a deflection about 
double the standard will be obtained, because two coils are 
connected between 3 and 4 in place of only one. When on 
4 and 5, the deflection will reverse, because the leads from 
K, S and K, W are crossed; but it will not be greater than 
the standard, because only one coil is connected between 
4 and 5. Between 5 and 6 a large deflection will be obtained 
as between 3 and 4 and for the same reason. Between 6 and 7 
little or no deflection will be obtained, because coil N is 
short-circuited, and hence there will be in it little or no drop. 

If a coil has poor or loose connections with the commutator 


34 


OPERATION OF 


bars, the effect will be the same as if the coil had a higher 
resistance than it should, and hence the deflection will be 
above the normal. In practice, after one has become used 
to this test, faults may be located easily and rapidly. It is 
best to have two persons, one to move C and the other to 
watch the deflections of G. 

59. Locating Short-Circuited Armature Coils. 
Where there are a large number of armatures to be tested, 
as, for example, in electric-railway repair shops, an arrange¬ 
ment similar to that shown in Fig. 13 is very convenient 




for locating short-circuited coils. A is a laminated iron 
core with the polar faces b, b (in this case arranged for four- 
pole armatures). This core is rectangular and is wound with 
a coil c that is connected to a source of alternating current. 
The core is built up to a length d, about the same as the 
length of the armature core. When a test is to be made, the 
core A is lowered near the armature, and when an alter¬ 
nating current is sent through c, an alternating magnetiza¬ 
tion is set up through the armature coils. This induces an 
electromotive force in each coil; and if any short circuits 
exist, such heavy local currents are set up that the short- 
circuited coils soon become hot or burn out, thus indicating 
their location. If the armature is rotated slowly, it is pos¬ 
sible to tell when a short-circuited coil comes under b, b 













































DYNAMO-ELECTRIC MACHINERY, PART 1 35 


by the increased current taken by coil c. If an armature 
with a short-circuited coil is revolved in its own excited 
field, the faulty coil will promptly burn out, so that this con¬ 
stitutes another method of testing for such faults. To cut 
out a short-circuited coil, temporarily disconnect its ends 
from the commutator, bend the ends back out of the way, 
tape them so that they cannot touch each other, and connect 
the two bars from which the coil ends were disconnected by 
a short piece of wire, or jumper. It is always better, how¬ 
ever, to replace the defective coil, because, if the turns are 
short-circuited on each other, the coil may persist in 
heating and thus damage other coils. 


REPAIRS 

60. Field Coils. —In case of accident to parts of the 
machinery, it is sometimes very convenient to make repairs 
on the spot, saving the time lost in sending the injured 
apparatus to the makers. There is usually no difficulty in 
rewinding field coils in a lathe. First weigh the old coil and, 
in removing the wire, note carefully the method of connect¬ 
ing, the size and insulation of the wire, and the insulation 
on the spool. Rewind the coil, using exactly duplicate 
features as nearly as possible, unless it is plainly evident 
that the conditions can be improved. 

If necessary to make a joint in the wire, the ends of the 
wires should be rubbed bright with fine sandpaper, twisted 
firmly together, and soldered with a hot iron, using a non- 
acid flux. Only solder enough should be left on the joint to 
make the connection between the wires solid. Remove all 
projecting ends or bits of solder, leaving a perfectly smooth 
joint and one occupying as little space as possible. The joint 
should then be well insulated with silk, cotton, paper, or 
adhesive tape. 

61. Armatures. —To rewind an armature, in whole or 
in part, is usually a much more difficult task, and if the job 
be of much importance, the advice or assistance of an 



36 


OPERATION OF 


experienced man should be obtained. If such work be 
attempted, proceed slowly, carefully noting connections, 
insulations, etc. in removing the old portion, and duplicate 
all these features, as nearly as possible, in the new winding. 

When complete, the binding wires should be replaced, and 
the winding tested for grounds, before connecting it to the 
commutator. It will be well, while replacing the winding, 
to make frequent tests for grounds or short circuits. 

When being replaced, binding wires should be subjected 
to a considerable tension, so that when they expand as the 
armature heats up they will not become loose. They should 
be soldered together quickly with a very hot iron, using as 
before only a non-acid flux. 

62. Balancing an Armature. —Many makers balance 

armatures by means of 
small masses of solder 
secured to the binding 
wires. If these binding 
wires are replaced the 
armature must be re¬ 
balanced in order that 
it may run without 
excessive vibration. 
For this purpose two 
iron or steel straight¬ 
edges or ways, as shown in Fig. 14, should be provided. 
These should be from i to f inch wide on the upper edge 
and from 12 to 18 inches long, depending on the weight 
and size of the armature to be balanced. They should be 
set level and parallel, and at such a distance apart that the 
journals of the armature shaft will rest on them. 

To balance the armature, it is placed on the ways, when it 
will turn over until the heavy side is beneath. A small weight, 
as a piece of solder, is then temporarily fixed to the upper part 
of the armature, which is then given a slight motion by the 
hand. It will settle in a new position, when another weight 
may be temporarily affixed to the armature, or a little of the 









DYNAMO-ELECTRIC MACHINERY, PART 1 37 


other weight removed, according to the judgment of the work¬ 
man. This operation should be continued until the armature 
shows no decided tendency to remain in any one position; 
the weights may then be permanently fastened in place. 

The method of repairing broken leads, connections, and 
the like may be readily seen from the nature of the fault. 
In any kind of repair, the object in view should be to replace 
the defective part so that it will be exactly as it was before 
being damaged, unless, as before stated, the conditions can 
be improved. 


OPERATION OF DIRECT-CURRENT 
MOTORS 


STARTING AND REGULATING DEVICES AND 
MOTOR CONNECTIONS 

63. The preceding discussion regarding the selection, 
installation, and care of electrical machinery applies with 
equal force to both generators and motors. Each may 
develop faults in insulation, open circuits, short circuits, etc. 
and each may cause trouble by sparking. The tests and 
the remedies in each case are practically the same. In the 
operation of motors, however, there are some features 
requiring special mention. Auxiliary apparatus is usually 
necessary with motors and a brief description of some of 
the most commonly used starting rheostats and speed control¬ 
lers will be given. 

64. When motors are operated on constant-potential cir¬ 
cuits, it is necessary to insert a resistance in series with the 
armature when starting the motor. In the case of a series 
motor, this starting resistance is also in series with the field. 
The resistance of a motor armature is very small, and that 
of a series-field is also small, so that if the machine were 
connected directly across the circuit while standing still, 
there would be an enormous rush of current, because the 
motor would be generating no counter electromotive force. 




38 


OPERATION OF 


For example, if a shunt motor of which the armature resist¬ 
ance is .1 ohm, were connected across a 110-volt circuit while 
the motor was at a standstill, the current that would flow 
momentarily would be 110^.1 = 1,100 amperes, the amount 
being limited only by the resistance and inductance of the 
armature. The rush of current through a series motor would 
not be quite so bad, as the field winding, owing both to its 
resistance and its inductance, would help to choke back the 
current. Inductance is the property of an electric current of 
producing a magnetic field around its conductor. This mag¬ 
netic field, which moves during any variation in the current 
strength, has the ability of inducing an electromotive force in 
the electric conductor itself as well as in any adjoining con¬ 
ductor. The effect of this induced electromotive force is to 
retard an increase or a decrease in the existing current strength. 
Hence, in a long conductor, such as a coil, a certain period of 
time is required for a current to reach its full strength, and 
when once reached, any variation is resisted by the inductance. 

65. The starting rheostat, or starting box, is a 
resistance divided into a number of sections and connected to 
a switch by means of which these sections can be cut out as the 
motor comes up to speed. When the motor is running at full 
speed, this resistance is completely cut out, so that no energy 
is lost in it. Starting rheostats are made in a great variety of 
forms and sizes, but the object is the same in all of them, that 
is, to provide a resistance that may be inserted when the motor 
is at rest and gradually cut out as the motor comes up to speed. 


SHUNT-MOTOR CONNECTIONS 

66. One method of connecting a shunt motor to constant- 
potential mains is shown in Fig. 15. The lines leading to the 
motor are connected to the mains through a fuse block D, 
from which they are led to a double-pole knife switch B. One 
end of the shunt field F and one brush are connected to ter¬ 
minal 1 of the motor; the other field terminal is connected to 
terminal 2 t and the other brush to terminal 3 , which is con¬ 
nected to one rheostat terminal. One side of the main 



DYNAMO-ELECTRIC MACHINERY, PART 1 39 


switch connects to terminal 1 ; the other side connects to 
terminal 2 and also through the starting rheostat C, to 
terminal 3. As soon as the main switch is closed, cur¬ 
rent will flow through the field F. When the rheostat arm 
is moved over, current will flow through the armature A and 
the motor will start; as the handle is moved over slowly to 
the last point the motor gradually attains its full speed. 

67. Fig. 15 shows connections for a motor having a 
three-point terminal 
block, one point for each 
line wire and a point for 
one field terminal, the 
other field terminal 
being brought directly 
to a brush. Modem 
motors are usually pro¬ 
vided with a separate 
terminal point for each 
field and armature lead; 
that is, a four-point 
block for a shunt motor. 

With such a block, the 
direction of current 
through either the field 
or armature can be re -1 
versed independently of 
the other, making it 
easy to reverse the 
direction of rotation of 
the armature. Usually 
such reversals are provided for in a controller so that a 
movement of the controller handle will reverse the direction 
of rotation. 

68. Methods of Connecting.— Fig. 16 shows three 
methods of connecting a shunt motor. The switches are 
shown as single pole for sake of clearness of diagram. In 
Fig. 16 ( a ), the shunt field is excited as soon as the switches 




























40 


OPERATION OF 


are thrown; this is the method used in Figs. 15 and 18. In 
Fig. 16 (b), the shunt field is not excited until the rheostat 
lever is thrown on to the first button, and when the lever is 
moved over to its full-on position the field current must flow 
back through the armature resistance; this is objectionable 
though as the resistance is usually low and the field current 




fte/eose Magnet 


0^9 ^aaaaaaaJ 




00000000 


/fe/ease Magnet 


O-U ^UaaaaaaJ 




(b) X> 



Fig. 16 


small, little harm results. On some rheostats, an auxiliary 
contact and path is made, as shown by a and the dotted line, 
to lead the field current around the armature resistance when 
the lever is in the full-on position. A wrong connection 
frequently made is shown in Fig. 16 (c). The shunt field, 
instead of being connected across the line, is connected 
directly across the armature terminals when the lever is on 



































DYNAMO-ELECTRIC MACHINERY, PART 1 41 


any of the contacts and hence receives only the voltage 
applied to the armature. 

69. Automatic, No-Voltage-Release, Starting 
Rheostat. —In Fig. 15, the simplest type of rheostat was 
shown in order to make the connections as clear as possible; 
but such rheostats are now used but little, because they 
afford no automatic protection. Suppose that the attendant 
shuts down a motor by opening the main switch, but forgets 
to move the rheostat arm back to the off-position. When 
the switch is again closed, the armature, not being protected 
by the rheostat resistance, may be injured by the rush of 
current. Again, if the power should momentarily go off the 
line, something that may 
frequently happen, the motor 
will slow down and possibly 
stop. When the power is 
thrown on again, the motor 
armature receives full volt¬ 
age unless the rheostat lever 
has, in the meantime, been 
moved to the full-off posi¬ 
tion. For these reasons, it 
is customary to arrange on 
almost every starting rheo¬ 
stat what is called an auto¬ 
matic, no-voltage-release mechanism, so that the 
rheostat handle will fly back to the off-position whenever the 
power is cut off from the motor. Fig. 17 shows a simple 
form of automatic rheostat made by the General Electric 
Company. The automatic feature consists of an electro¬ 
magnet A in series with the motor field. The lever C is 
moved over against the action of a coiled spring, and is held 
at the full-on position by the attraction of magnet A for the 
armature B. Fig. 18 shows the rheostat connected to a 
motor. If the current supply be interrupted, the current in 
coil A will gradually decrease as the motor slows up and 
ihe counter electromotive force falls. The pull of the 








42 


OPERATION OF 


magnet becomes weaker, until finally the armature# is released, 
and the arm flies back to the off-position. With such a rheostat 
the proper way to stop the motor is to open the main switch 
and let the rheostat take care of itself. 

The automatic release magnet, instead of being connected 
in series with the shunt-field circuit, is sometimes connected, 
with or without a resistance in series, directly across the main 
circuit, so that the release coil is excited independently of the 



Fig. 18 Armofure 


shunt-field current. This is nearly always the case with 
rheostats for series-wound motors and some manufacturers 
adopt this plan for all their no-voltage magnets. 

70. Overload protection is also incorporated with start¬ 
ing devices by arranging a magnetic latch to release the switch¬ 
ing device if the current in the armature becomes too great 
for safety. A release of this sort that operates by demagnet¬ 
izing the low-voltage release magnet is not effective against 







































DYNAMO-ELECTRIC MACHINERY, PART 1 43 


overloads while starting a motor, since it affords protection 
only when the switching device is in the running position. 
The starter shown in Fig. 19 has an overload release that is 
effective whenever the starting lever b is over any of the 
resistance contacts. 

The connections are shown in Fig. 20, in which similar letters 
indicate similar parts. These connections are the same in 
principle as those shown in Fig. 16 ( b ). The wrong connec¬ 
tions shown in Fig. 16 (c) are made by interchanging the wires 
coming to the two binding posts on the rheostat; that is, the 
one marked Arm, Fig. 20, and that to the left of it. The low- 
voltage release coil a 
is energized when the 
starting lever b rests 
over any of the con¬ 
tacts 1,2,3, etc., pro¬ 
vided the overload 
release lever c remains 
in the position shown 
in both illustrations. 

The overload coil d is 
connected in series 
with one of the line 
conductors, so that all 
current to the motor 
must flow through this 
coil, the release lever 
c, starting lever b, starting resistance, to the armature as well as 
to the no-voltage release a, and the shunt field. 

If the motor is so overloaded that the current required 
exceeds a predetermined amount, the core of the electromagnet d 
is pulled upwards and swings the small catch to the left, thus 
releasing a pin held by it and allowing the lever c to be pulled 
up by a spring and open the circuit. The magnet a is 
then demagnetized and releases the lever b, which returns to 
the off-position. The arrangement on this rheostat is such that 
the arm c cannot be closed unless the lever b is in the off-position. 
The field connections shown by the full lines are those of a 













44 


OPERATION OF 


shunt motor; the series field of a compound-wound motor is 
connected as shown by the broken lines, the direct connection, 
indicated by the full line between the points e and /, in that 
case being omitted. 


71 • Starting and Stopping a Motor. —Starting a motor 
with a starter similar to that shown in Figs. 18 and 19 is accom¬ 
plished by first closing the line switch and then moving the 

starting lever over 
the row of resistance 
contacts, frequently 
called steps or points. 
The movement should 
be slow enough to 
allow the motor speed 
to accelerate smooth¬ 
ly. On the point at 
the extreme right, the 
lever is held by the 
low-voltage retaining 
magnet; this is the 
point on which the 
lever remains while 
the motor is running 
and is therefore called 
the running point. 
The lever will not re¬ 
main at rest on any 
intermediate point 
and must not be held there longer than necessary for the speed 
to pick up. 

Stopping a motor with any of the starters illustrated is 
generally best accomplished by opening the line switch or 
circuit-breaker. Such a circuit-opening device should be a 
part of every motor installation, as indicated in Fig. 20. On 
opening this switch the motor speed will decrease until the 
magnets release the switching devices, which will automatically 
return to position for the succeeding start. 



Fig. 20 




















DYNAMO-ELECTRIC MACHINERY, PART 1 45 


72. Automatic Starting Switches. —The automatic 
starter consists of a combination of switch units which close 
automatically, each successively cutting out a section of 
resistance when the motor has accelerated to the proper point. 
One of these switch units, or contactors, is shown in Fig. 21. 
These contactors are made especially for starters and are 
variously called series switches, series contactors, and magnetic 
lock-out switches, the last name referring to the method by which 
their operation is delayed. 

The magnet coil a is series- 
wound, that is, it carries 
the armature current, and 
the switch is so made that 
it cannot close while the 
current through this coil 
exceeds the predetermined 
value for which the adjust¬ 
ments are made. 

Two return paths are 
provided for the magnetic 
flux outside the magnet 
core, one through a mag¬ 
netic shunt b and the other 
through a vertical iron 
strip c attached to the 
moving element. The mag¬ 
netic pull across the air gap 
d tends to close the con¬ 
tacts e, while the magnetic 
pull between the lower end of the iron strip c and the pole / 
tends to hold the contacts open. A copper band around the 
magnetic shunt b prevents sudden changes of flux, and the 
cross-section of this shunt is so small that it becomes highly 
saturated when the magnetizing force is high. When the cur- 
. rent in the coil exceeds the adjustment, enough flux passes 
through the iron strip c to hold its lower end against the pole /. 
But when the current decreases, the flux through the strip c 
becomes less until the pull across the gap d becomes superior, 



444—4 






















4G 


OPERATION OF 


and the moving element turns on its pivot g, closing the contacts e 
and causing the lower end of the strip c to swing outwards. 


73. A self-locking nurled nut h, Fig. 21, serves to adjust 
the length of the gap d and thus adjusts the current at which 
the switch closes. The contacts e } in closing, touch first near 
the tips and then rock back toward the heels by turning the 
arm to which the moving contact is attached around the pivot i. 

In thus turning, the 
spring j is compressed 
so that when the con¬ 
tacts open, the rock¬ 
ing motion is reversed 
and the circuit is 
opened at the tips of 
the contacts. To pre¬ 
vent the formation of 
destructive arcs on 
opening, arc shields, 
made of refractory 
material, are em¬ 
ployed. 

When several of 
these contactors are 
properly grouped in a 
starter and adjusted, 
they will close auto¬ 
matically, as soon as 
the line switch is 
closed, in the sequence of time necessary to bring the motor 
up to speed. In this case, the delay in operation of each switch 
depends entirely on the current in the circuit. 



Fig. 22 


74. Speed-Regulating Rheostats.—Speed-regulating 
rheostats, often called speed regulators, are very similar 
in construction, in appearance, and in connections to starting 
rheostats except that regulators, owing to their greatly increased 
carrying capacity, are much the larger. The chief difference, 
however, between the two is that while a starting rheostat has 




















































DYNAMO-ELECTRIC MACHINERY, PART 1 47 


resistance so proportioned as to carry the starting current 
required by the motor armature for only a few seconds, usually 
not over 15, a speed regulator has resistance designed to carry 
the armature current continuously. The starting rheostat 
lever should, therefore, never be held longer than 2 or 3 seconds 
on any step except the last, on which the resistance is all cut 
out. The speed-regulator lever is usually arranged to be held 
automatically on any desired step. Fig. 22 shows the con¬ 
nections of an automatic speed regulator for a shunt motor. 
The segment S, which turns with the lever L, is fixed in any 
required position by a pawl or catch t engaging one of the 
notches in 5 by the action of the magnet M. The notches are 
so distributed that each corresponds to the position of the lever 
squarely over one of the contact segments c, c. If the voltage 
of the circuit fails or if the switch is opened, the magnet M 
releases the pawl t and the lever flies back to its initial position. 
In this type of regulator, the contact segments c, c are renew¬ 
able and may be easily replaced if they become worn or burned. 


SERIES- AND COMPOUND-MOTOR CONNECTIONS 

75. Connections for shunt motors have been discussed 
first because they are the most complicated and the most 



necting the other field windings will then be easily derived. 
































































48 


OPERATION OF 


Fig. 23 shows simple diagrams of connections for series- and 
compound-wound motor starters with automatic underload 
release. The release spool 5 of a series-motor starter is usually- 
connected directly across the circuit with a resistance r in 
series unless the voltage of the circuit is very low, in which 
case the resistance is omitted. 

Since its field helps to choke back the starting current, a 
series motor does not require so large a starting resistance as a 
shunt motor. 


REVERSING THE DIRECTION OF ROTATION 

76* If the current in either the field or the armature of a 
motor is reversed, the direction of rotation will be reversed; 
but if the current in both the field and armature be reversed, 
the direction of motion will remain unchanged. A series 
motor will, therefore, run in the same direction, whatever the 

direction of the current through 
the machine. Reversing the line 
connections to terminals a, b } 
Fig. 24, simply reverses the cur¬ 
rent through both armature and 
field and does not change the 
direction of rotation. In order 
to reverse the motor, either the 
armature terminals c, d must be interchanged, so as to reverse 
the current through the armature, or the terminals d , b must be 
interchanged, so as to reverse the current through the field. In 
mine locomotives and other electric-railway work, the motors 
are usually reversed by reversing the direction of the current 
through the armature, that of the current through the field 
remaining unaltered. 

To reverse a motor while it is running, it is necessary to 
insert a resistance in the armature circuit so as to reduce the 
speed or even bring the motor to rest before reversing the cur¬ 
rent through the armature. The counter electromotive force 
that the motor was generating just before reversal becomes 
an active electromotive force and helps to make the cur¬ 
rent flow through the armature as soon as the direction of the 







DYNAMO-ELECTRIC MACHINERY, PART 1 49 


flow is reversed. This causes a very large current to pass until 
the motor starts to turn in the opposite direction and builds up 
a reversed counter electromotive force; hence, the necessity of 
reducing speed or even stopping the motor before reversing it. 

77. Reversing Switches. —In order to reverse the cur¬ 
rent in a motor armature so as to reverse its rotation, a revers ¬ 
ing switch is placed 
in the armature cir¬ 
cuit. Fig. 25 shows 
three common types 
of switch. That shown 
at (a) consists of two 
metal blades a, b 
hinged at c and d and 
connected together by 
an insulating cross¬ 
piece e. The blades 
can be swung from 
the position shown in 
the figure to that indi¬ 
cated by the dotted 
lines, by the rod /. 

The points g and k are 
connected by a wire 
conductor a s indi¬ 
cated by the dotted 
line. In the first posi¬ 
tion, c and d are con¬ 
nected to g and h, 
while in the second 
position they are con¬ 
nected to h and k, 
which reverses the 

current in the arma- FlG ' 25 

ture. Fig. 25 (6) shows an ordinary double-pole, double-throw, 
knife switch used as a reversing switch. The middle clips are 
connected to the armature, while the top and bottom clips are 
































50 


OPERATION OF 


cross-connected, so that when the switch is thrown up, the 
current in the armature is in one direction, and when it is 
thrown down, this current is reversed. The reversing switch 
shown in Fig. 25 (c) is of the cylinder type, and is used very 
largely for railway and hoisting motor controllers. 

The upper portion of 
this figure is a plan view 
of the cylinder showing 
the contact pieces e , /, g, 
h, k, l. The cylinder is 
usually made of wood or 
else pieces are insulated 
from it and they serve 
only to change the con¬ 
nections between contact 
fingers represented at a, 
b,c,d. The lower portion 
of the figure shows the 
end of the cylinder and 
the position of one finger 
d and contact pieces/ and 
Z, the other fingers and 
contact pieces being in 
direct line with those 
shown. When the cylin¬ 
der is rotated so that the 
fingers rest on plates e 
and /, as indicated by the 
dotted line 1, the current 
entering at a takes the 
path a-e-b, and, passing 
through the armature, traverses c-f-d. When the drum is turned 
so that the fingers rest on the contacts g, h } k , Z, as indicated by 
the dotted line 2, the path becomes a-g-k-c and passing through 
the armature in the reverse direction then traverses b-h-l-d. 

78. Shunt Motor With Reversing Switch. —Fig. 26 
shows connections for a shunt motor with reversing switch R. 



Shuntfle/d 

Fig. 26 




































DYNAMO-ELECTRIC MACHINERY, PART 1 51 


The field is excited from the mains as soon as the rheostat arm 
is placed on the first point, and remains excited in the same 
direction regardless of the position of the reversing switch. 


CONTROLLERS 

79. For motors that have to be stopped, started, and 
reversed frequently, special types of starting devices are used. 
These are generally called controllers. For electric-railway 
work, these controllers are sometimes quite complicated, being 
designed not only to cut resistance in or out, but also to make 
various combinations of the two or more motors used on a car. 
An explanation of railway controllers will be found in Haulage , 
Part 3, so they will not be considered here. Controllers some¬ 
what similar to those used on 
electric cars and locomotives are 
also used for stationary work, but 
when so used they are generally 
required to control but one 
motor, and hence are designed 
to simply cut resistance in or 
out and not to make series and 
parallel combinations. 

The use of series motors in 
places calling for heavy service 
has resulted in the development 
of a large number of controlling 
devices especially adapted to 
work of this kind. For such 
service, the motor must be capable of being stopped, started, 
and reversed quickly, and the controller must be of simple and 
substantial construction. 

Figs. 27 and 28 show typical crane controllers for motors of 
5 and 30 horsepowers, respectively. All speed regulation is 
by armature control, and in each case the motor can be operated 
in either direction, depending on the way in which the handle 
is moved from the off-position , or the position in which the 
motor circuit is open. Both controllers are shown so arranged 






52 


OPERATION OF 


that the operators must stand near them, but each can be 
arranged for operating from a distance by means of ropes or 
rods. Thus, the smaller controller can be operated by ropes 

from the floor underneath 
the crane, and the larger one 
can be mounted outside the 
crane cage and operated by 
means of a bell-crank and a 
connecting-rod. 

80. The larger controller, 
Fig. 28, has two pairs of 
moving contacts, those of 
each being connected in par¬ 
allel. A magnetic blow-out 
coil, Fig. 29, mounted near 
each contact and connected 
in series with it sets up a 
strong magnetic field be¬ 
tween the poles a directly 
through the space where arcs 
form between the moving 
and the stationary contacts; 
the arcs are thereby prompt¬ 
ly disrupted, thus minimizing 
injury to the contacts. 

81. Drum Controllers. 

Drum controllers arranged 
for adjusting armature-con¬ 
trol resistance only are much 
used in connection with 
heavy crane and hoist service, 
as well as for railway work. 
The upper portion of Fig. 30 
shows external connections of such a controller, with motors 
having different types of field windings. The lower portion of 
the figure shows the internal connections. The stationary fingers 
in the controller are represented by the circles in the center; 



Fig. 28 





















DYNAMO-ELECTRIC MACHINERY, PART 1 53 


fingers R b , R 4 , etc., are connected with resistor terminals simi¬ 
larly lettered and the other terminals with the armature and 
the line. The moving drum segments are represented by small 
rectangles, and the drum positions, or steps, are indicated by 
the vertical dotted lines 1, 2, 8, 4, 5 through these rectangles. 
Blow-out coils are provided where necessary to prevent 
destructive arcing. 

82. Turning the drum either way from off-position moves 
segments under the fingers in a way to complete a circuit 
through the motor and to cut out the control resistance step 
by step until all is out on the fifth step. The direction of 
current in the motor armature and, consequently, the direction 



of rotation of the motor depends on the direction in which the 
drum is turned. 

83. Automatic Controllers. —Automatic controllers 
composed of magnetically operated switches, or contactors, 
are made for many kinds of industrial service. Shunt con¬ 
tactors are generally used for speed-control purposes, and the 
interlocking contacts are so interconnected that each switch 
controls the operation of the next switch in the series. By 
means of these switches and suitable safety devices, an operator 
with a simple master controller can exercise perfect control 
over large motors in difficult processes where starting, stopping, 
and reversing are frequently required. The master controller is 
a simple switching device to control the exciting current of the 
magnets on the contactors. 











Fig. 30 


54 























































































































DYNAMO-ELECTRIC MACHINERY, PART 1 55 


SELF-STARTING RHEOSTATS 

84. Sometimes is is necessary to have a starting rheostat 
arranged so that it can be controlled from a distant point, in 
which case the box has to cut out its starting resistance auto¬ 
matically. The most common method of accomplishing this 
is to provide the rheostat with a solenoid that, when energized, 
moves the contact arm and cuts out the resistance. 



85. Fig. 31 shows an example of an automatic starter as 
used to control a motor that operates a pump supplying water 
to a tank. The switch d controlling the motor is located at 
the tank, which may be some distance from the motor and its 
rheostat. The switch is opened and closed at high- and low- 
water marks by a float in the tank. The automatic closing of 
the switch d when the tank is empty operates the solenoid 



















































56 


OPERATION OF 


switch e at the pump house, thereby raising its core and making 
contact between studs /, g. This closes the main circuit and 
energizes another solenoid a, which draws up its core, moving 
the arm b slowly over the contacts, its speed being controlled 
by the action of the oil dashpot c. By this means, the motor 
is started and brought gradually up to speed. The moment 
arm b leaves its lowest position, carbon points at k separate, 
throwing an incandescent lamp in series with coil e\ and when 
the arm reaches its highest position, it causes the carbon points 
at h to separate, thus throwing another lamp in series with 
coil a. The lamps and their connections are not shown in the 
figure. When the magnet cores are in the lowest positions, a 
considerable current in each coil may be required to raise them; 
but when the cores have been drawn well into the coils, a small 
current in each is all that is necessary and connecting a lamp 
in series with each magnet not only prevents overheating the 
wire but also saves current. 


STARTING A DIRECT-CURRENT MOTOR 

86. When installing a motor in an isolated place where a 
voltmeter is not available, it is well to permanently connect an 
incandescent lamp across the circuit near the motor so as to 
supply a ready means of ascertaining whether power is on the 
line at any time. By using a key socket, or receptacle, the 
lamp may be switched off when not needed. 

Before attempting to start the motor see that there is power 
on the line and then close the main switch. This may or may 
not allow a current to flow through the motor fields, according 
to the kind of winding and the method of connecting. Move 
the lever of the starting rheostat quickly and squarely to the 
first contact segment and let it stay there for 2 or 3 seconds. The 
motor should start at once and begin to increase in speed. Move 
the lever on from segment to segment, stopping on each but 2 or 
3 seconds, until the full-on, or short-circuit, position is reached, 
where the lever should be firmly held by the retaining magnet. 
During this process, the motor speed should have gradually 
increased to full speed, the total time required to accelerate to 



DYNAMO-ELECTRIC MACHINERY, PART 1 57 


full speed being usually about 15 seconds. Do not hold the 
lever longer than indicated on any contact, unless the starting 
resistance be intended also for speed control. If the motor 
does not start when the lever is on the first contact, move 
quickly to the second. If still no start is made, move to the 
third and, if the machine fails to start, immediately open the 
main-line switch and look for the cause of the failure. The 
failure may result from any one or more of several causes, 
namely: 

1. Wrong connections, of which the most commonly 
occurring example for shunt fields was indicated in Art. 68. 
Make sure that the shunt field obtains the full voltage when the 
lever is on the first step, and that the poles are magnetized. 

2. An overload on the motor; when a motor is first installed, 
the current required to start its load as well as the running 
current after obtaining full speed should be ascertained. An 
ordinary motor intended for continuous service should not be 
expected to start a load requiring more than double its rating 
in amperes. This rating is usually stamped on the name 
plate. Motors intended for intermittent service, such as rail¬ 
way and hoisting work, are designed to start with almost any 
load up to what would actually stall the armature. 

3. An opcni circuit due, possibly, to a defective switch, 
a broken wire or poor connection in the starting box, or the 
brush not making good contact with the commutator, or an 
open circuit within the motor itself. 

4. A short circuit, which will nearly always make its 
presence and possibly its location known. Among the more 
common sources of short-circuiting are: short-circuited arma¬ 
ture coils; short-circuited commutator; short-circuited field 
coils; brushes in the wrong position. If the armature coils or 
commutator are short-circuited, the machine may start and 
turn over part way and stop again. With a series field-coil 
short-circuited, the armature will start only under a heavy 
current, with accompanying sparking, and will acquire a high 
rate of speed. A wrong position of the brushes will usually 
be indicated by violent sparking. The correct position may 
be found by trial if not already marked on the frame. 

























OPERATION OF DYNAMO- 
ELECTRIC MACHINERY 

Serial 839B (PART 2) Edition .? 


DIRECT-CURRENT GENERATORS IN 
COMBINATIONS 


GENERATORS IN SERIES 

1. Generators are seldom run in series. As in the case of 
a series-connection of battery cells, a series-connection of 
generators adds their pressures but does not change the total 
current output. Occasionally in a long direct-current trans¬ 
mission line in the United States, usually a line carrying 
current for an electric railway, a series generator is connected 
in series with the main generators in the power house to 
raise, or boost, the voltage, such a generator being called a 
booster. The booster is driven at a constant speed and 
whenever no current is flowing out to the line, the booster, 
being series-wound, has no field strength, and hence gener¬ 
ates no electromotive force; when a large current is flowing, 
the booster has a very strong field, and hence generates 
an electromotive force, which is added to that of the main 
generators. The amount of electromotive force generated 
by the booster depends on the amount of current flowing 
through its field and out to the line. In other countries, 
especially in some plants in Europe, a number of direct- 
current generators are connected in series to produce a pres¬ 
sure of several thousand volts for transmitting current over 
long distances. 

COPYRIGHTED BY INTERNATIONAL TEXTBOOK COMPANY. ALL RIGHTS RESERVED 




60 


OPERATION OF 


Generally speaking, series-wound, shunt-wound, or com¬ 
pound-wound generators may be run in series with very little 
difficulty, the series field-winding, when there is one, always 
being connected in series with the line. But in most cases, 
the demand is for large current output rather than high volt¬ 
age, and to increase the current requires parallel connection. 


GENERATORS IN PARALLEL 

2. Direct-current generators are frequently operated in 
parallel, the connections being as shown in Fig. 1, where, in 
order to make the connections as simple as possible, no field 
windings are shown. Each machine generates the same 



voltage, and the pressure between the lines is the same as if 
a single machine were used; but the current delivered to the 
external circuits is the sum of the currents delivered by the 
several machines; hence, the outputs are combined by adding 
the currents from the several generators. Each machine 
delivers current through its main switch M or M' to the heavy 
conductors, or bus-bars, C, D, connected as shown. Like 
terminals of each machine must be connected to the same 
bus-bar. 























DYNAMO-ELECTRIC MACHINERY, PART 2 61 


It is not so easy a matter to operate machines in parallel 
as in series. It is evident that the voltage of each machine 
must be kept at the proper amount if the combination is to 
operate satisfactorily; for, suppose that the electromotive 
force of B, Fig. 1, should fall below that of A, then A will 
send current through B and run it as a motor, and B will 
thus be taking current from A instead of helping it feed into 
the line. 

Series generators are seldom run in parallel; shunt gener¬ 
ators are sometimes, but compound-wound generators are quite 
frequently so operated. 

SERIES GENERATORS IN PARALLEL 

3. Suppose two series generators to be connected as shown 
in Fig. 2 and assume that each machine is delivering one- 



half the current required by a certain load. As long as the 
two machines generate exactly the same voltage, they will 
continue to share the load equally; but if the voltage of one, 
say A , drops slightly, owing to reduced speed or other 
cause, that machine will at once cease to furnish its full 
share of the load, thus throwing more than one-half the load 
on the other machine B . Both machines being series-wound, 
the field of A will be weakened, thus still further decreasing 
its voltage, and the field of B will be strengthened until 
soon A will be overpowered, its current reversed, and it will 
be run as a motor with its direction of rotation reversed. 
This may result in considerable damage. The action of the 
two or more series machines connected as described in 
parallel, will, therefore, be very unstable. 


444—5 













62 


OPERATION OF 


4. Equalizer Connection. —The unstable condition 
just referred to can be remedied by connecting the inner 
ends of the series-fields of the two machines—that is, the 
ends connected to the dynamo brushes—by a low-resistance 
conductor, commonly called an equalizer. Fig. 3 shows 



the same connections as Fig. 2, except that an equalizer E 
has been added connecting the points c and d where the 
series-coils are attached to the brushes; e and / are the posi¬ 
tive terminals of the machine. If, for any reason, the 
machine B at any time delivers a greater current than A> 
part of this current will flow to the + line through the coil df , 
and part will take the path d-c-e through the series-field 
coil c e of machine A. The result is that part of the current 
delivered by B helps to keep up the field excitation of A, 
thus bringing up its voltage and equalizing the load between 
the machines. On the other hand, if A for any reason 
delivers the greater current, part of its current will flow 
through the path c-d-f and strengthen the field of B in the 
same manner. 


SHUNT GENERATORS IN PARALLEL 

5. Connections for two shunt generators are shown in 
Fig. 4. The usual arrangement is to use double-pole single¬ 
throw switches instead of the single-pole switches M,N and 
MN' shown here for sake of clearness of diagram. 

If these two machines were operating in parallel, each 
supplying one-half the required current and one, say A, 
owing to a speed drop or any other cause, should reduce its 
voltage, and fail to supply its half of the current, more than 













DYNAMO-ELECTRIC MACHINERY, PART 2 63 


one-half the load would then be thrown on B. As the load 
on a shunt generator decreases, its voltage rises; and as the 
load increases, the voltage falls; therefore, the tendency 
would be for the voltage of A to increase and for B to decrease 
until the two were again equal. Shunt generators are, there¬ 
fore, well adapted for parallel operation. 

6. Starting Shunt Generators in Parallel. —All 

switches should be open while the machines are standing 
idle. In starting, both generators, having their switches still 
open, may be brought up to full speed if desired. One 
machine, say A, Fig. 4, is first built up and thrown into cir¬ 



cuit as follows: Close its field switch L and adjust its field 
strength by means of rheostat r until the voltmeter V 
indicates the proper electromotive force; close the main 
switches M,N, and machine A will then supply all the 
current, if it is capable. Now, close the field switch L' of 
machine B and adjust the rheostat r f until the voltmeter V 9 
indicates 1 or 2 per cent, higher electromotive force than 
voltmeter V, and then close the main switches Hf, N'. The 
two machines, if of the same capacity, should then each 
























64 


OPERATION OF 


supply very nearly the same current, as indicated by the 
ammeters C, C'. The division may be made as desired by 
adjusting one or both rheostats r, r'. 

If the shunt field of the second machine is connected 
directly across the line, it will build up much more rapidly 
when its field switch is closed, because its field is then sub¬ 
jected to the full electromotive force of the other machine. 

Any number of shunt generators may thus be operated in 
parallel, each succeeding one being started and thrown into 
circuit by the process described for machine B. 


COMPOUND-WOUND GENERATORS IN PARALLEL 

7. Since a compound-wound generator is a combination of 
a series and a shunt generator, the arrangement for parallel 



running of such machines is a combination of the two pre¬ 
ceding ones. The connections are shown in Fig. 5, where 






















DYNAMO-ELECTRIC MACHINERY, PART 2 65 


the parts are lettered as in the two preceding figures. A 
switch E has been here added to the equalizer. 

The machines are usually first adjusted separately by means 
of the shunts s, s' around the series-fields, so that at the same 
proportion of its load each will give as nearly as possible the 
same voltage, that is, the voltages must be made to agree at 
no load and at full load and they must also agree as nearly 
as possible at one-fourth load, one-half load, three-fourths 
load, etc. The machines need not necessarily be of the same 
capacity. The figure shows two methods of connecting the 
shunt field, one, known as short shunt, directly across the 
brushes, as shown by full lines, and one, known as long 
shunt, across the machine terminals, as shown by dotted lines 
re and r '/. It makes little difference which method is used. 

In Fig. 5, there are two paths for current to pass from the 
positive brush of either machine to the positive bus-bar; one 
through its own series-field and one through the equalizer 
and the series-field of the other machine. If the resistance 
of the equalizer were zero, it is evident that the division of 
the current from either positive brush through the two paths 
would be inversely proportional to the resistance of the paths, 
that is, the path having the higher resistance would carry 
the smaller current. The division of the current between 
the two series-fields may, therefore, be made as desired by 
adjusting the resistance of one or both paths c-k and d-l. 

8 . Adjusting the Division of Load.—Suppose that 
two generators A and B, Fig. 5, of unequal capacities are to be 
adjusted so that they will share the load at all times very 
nearly in proportion to their total capacities. The compound¬ 
ing of each machine is first adjusted, as explained in Art. 7, 
by adjusting its series-shunt ^ or If when the two are 
connected in parallel one of them, A, supplies more than its 
share of the current, the resistance of the path c-k through 
the series-field coil of A should be increased until the divi¬ 
sion of the load is correct. The resistance of the series-field 
coil of a generator is usually very small, so that only a very 
slight addition will be needed in any case. The necessary 


66 


OPERATION OF 


resistance may be obtained by using a longer lead between e 
and k or possibly by inserting iron or German-silver washers 
under a terminal lug. It is useless to try to adjust the divi¬ 
sion of load by adjusting the series-field shunts s, s', for when 
the machines are connected in parallel, adjusting either shunt 
affects both machines alike. The remedy for improper divi¬ 
sion of load in such cases is to insert a very slight resistance 
in series with the series-field coil of the machine taking more 
than its share. 

For the most successful parallel operation, generators should 
be of the same design and construction and should possess as 
nearly as possible the same characteristics; that is, each should 
respond with the same readiness, and to the same extent, to 
any change in its field excitation. Any number of such 
machines may be operated in parallel. 

9. The usual practice is to connect the equalizer and the 
series-field to the positive brush, though this is not necessary; 


fie/d Rheos/af 

do. / 7~o l /ne 



they must, however, both be connected to the same brush. 
The resistance of the equalizer should be as low as possible 
and it must never be greater than the resistance of any of the 
leads from the generators to the bus-bar; that is, ek or fl, Fig. 5. 














































DYNAMO-ELECTRIC MACHINERY, PART 2 67 


In some cases, the equalizer wire is run directly between 
the machines; but often in lighting or small railway stations, 
a third wire is run to the switchboard and there connected to 
an equalizer bar, as shown in Fig. 6, which represents a very 
common arrangement, triple-pole switches M , M' being used, 
the two outside blades being for the + and — leads, respect¬ 
ively, and the middle blade for the equalizer. There is a 
difference of opinion as to whether it is better to run the 



Fig. 7 

equalizer to the switchboard or to run it directly between the 
machines, as in Fig. 5; but the most recent practice favors 
running it directly and placing the equalizer switch near the 
machine. This undoubtedly shortens the connections and 
permits better regulation. In such cases, the equalizer switch 
is usually mounted on a stand near the machine. 

10. In some railway plants, where large generators are 
used, the main switch b> Fig. 7, is placed on the stand near 
the machine, alongside the equalizer switch a . These two 






















































68 


OPERATION OF 


switches are at practically the same potential, and there is no 
objection to placing- them near each other. In case this is 
done, one of the bus-bars, together with the equalizer bus, is 
placed under the floor near the machines. This shortens the 
connections considerably and makes the equalization of the 
load closer. It also simplifies the switchhoard connections 
and avoids crowding on the generator switchboard panels. 

In Fig. 7, the main connections only have been shown, 
the shunt-field coils and all minor connections being omitted. 
The + leads from all the machines connect to the + bus¬ 
bar under the floor. If the machines are of equal capacity, 
these leads should all be of the same length in order to 
secure close equalization. In the case of machines 2 and 3 , 
the leads are doubled back as shown at d in order to make 
them of the same length as those from the more distant 
machines. 

11. Starting Compound-Wound Generators in 
Parallel.—The general method of starting any machine, say 1, 
Fig. 7, and throwing it in parallel with others already run¬ 
ning is as follows: See that all switches on the generator 
panel of the machine are open; then bring the generator up to 
speed. Now, close the equalizer switch a , the + switch b , 
and lastly the shunt-field switch on the generator panel. As 
the series-coils of all the machines are in parallel, some of 
the current from the other machines will flow through the 
series-field of machine 1 , causing it to pick up rapidly; and 
since its shunt-field circuit is also closed, the machine will 
soon come up to full voltage. Adjust the voltage by means 
of the shunt-field rheostat until it is 1 or 2 per cent, higher 
than that of the other machines and then close the negative 
switch e> thereby completing the operation. 

This method of procedure applies to the case where the +, 
—, and equalizer switches are independent of one another, as 
is usually the case in modern installations. When triple-pole 
switches are used, as in Fig. 6, all three must, of course, be 
closed together after the machine has been allowed to pick 
up its field and has had its voltage adjusted. After throwing 


DYNAMO-ELECTRIC MACHINERY, PART 2 69 


a machine in parallel, its load is adjusted by varying the 
field excitation. In case the machine is provided with a cir¬ 
cuit-breaker, as is nearly always the case on modern switch¬ 
boards, the circuit-breaker should be closed before the main 
switch, so that, if any rush of current occurs when the main 
switch is closed, the circuit-breaker will be free to act and 
disconnect the machine. 

12. Main and Equalizer Cables.—In connecting the 
machines to the switchboard, cables of ample capacity should 
be used. For most cases, it will be sufficient to allow from 
1,200 to 1,500 circular mils per ampere. Sometimes, an 
allowance as low as 1,000 circular mils per ampere is made, 
but the better practice is in favor of a more liberal cross- 
section. For very large currents, it is advisable to use two 
or three cables in parallel rather than a single large cable, 
as better radiating facilities are thereby provided. The cables 
leading to the equalizer bus should be of the same size as 
the main cables. _ 


ALTERNATING-CURRENT MACHINERY 


ALTERNATORS IN COMBINATIONS 

13. As a rule, alternating-current machinery does not 
require as much care and attention as direct-current machinery. 
Many suggestions made for selecting direct-current machines, 
and nearly all the remarks relating to their installation, apply 
with equal force to alternating-current machines. Starting 
or stopping a single alternator with its exciter, is usually a 
very simple matter; but these machines are frequently 
required to operate in combination, which necessitates special 
consideration. _ 


ALTERNATORS IN SERIES 

14. Alternators cannot be run in series unless their arma¬ 
tures are rigidly connected by being mounted on the same 
shaft, so that the electromotive forces generated by the two 
machines will be equal and in synchronism. But this method is 







70 


OPERATION OF 


obsolete, as high pressures can more conveniently be generated 
either with a single alternator or with an alternator and 
transformers. 


ALTERNATORS IN PARALLEL 

15. Alternators can be operated in parallel, although such 
operation is, as a rule, somewhat troublesome. This is 
especially the case if they are very different in size and design. 
They are usually connected to bus-bars through intervening 
main switches in much the same way as direct-current 
machines. If the alternators are compound wound, equal¬ 
izing connections should be used; but many are operated 
with a separately excited field only, and no equalizing con¬ 
nection is necessary, the whole scheme of connection corre¬ 
sponding more nearly to the running of shunt-wound machines 
in parallel. 

In order to operate alternators successfully in parallel, it is 
necessary that the electromotive forces developed by the two 
machines have the same frequency and are in phase, or in step, 
with each other. Two alternators have the same frequency 
when they reverse their polarities at the same instant. They 
are said to be in phase when the terminals are alive with 
positive or negative potential at the same instant. Clearly, 
it is not sufficient that the polarities change at the same instant, 
if they are not in phase; that is, if corresponding terminals 
have not the same polarity. Alternators are in synchronism 
when their currents have the same frequency and are in phase 
with each other. 

16. Synchronizing:. —The state of synchronism may be 
ascertained by means of what are called synchronizing: 
lamps connected as shown in Fig. 8, where T, T' represent 
small transformers having their primary coils connected to 
the alternators, similar terminals 1,1' being connected to 
similar sides of the machines. The secondary coils are con¬ 
nected in series through a pair of lamps l, l and a plug 
switch m. Suppose the two alternators to be operating at 
the same frequency and in phase and consider the instant 



DYNAMO-ELECTRIC MACHINERY, PART 2 71 


when the pressures between the alternator leads are maximum 
for each machine, tending to cause a maximum current to flow 
through the transformer primaries from 1 to 2 and from 
1' to 2', respectively. This will cause maximum pressures to 
be set up in the secondaries tending to force current from 4 to 3 
and from 4 ' to 3', respectively; but, as these pressures are 
opposed to each other in the secondary circuit, as shown by the 
arrows, no current will flow and the lamps will be dark. If the 
currents are of the same frequency but opposite in phase, 
the current from 1 to 2 will be a maximum at the same time as 



from 3' to 4' at the same instant; that is, the currents in the 
secondaries will now be in the same direction and the lamps 
will burn at full brilliancy. The one instant taken is illus¬ 
trative of what occurs every instant throughout a cycle. If 
the currents are of the same frequency and in phase, the 
pressures of the two secondary coils will neutralize each 
other and the lamps will be dark; if the machines are exactly 
opposite in phase, the secondary pressures will supplement 



































72 


OPERATION OF 


each other and the lamps will glow at full brilliancy. As the 
machines approach synchronism, the lamps will become alter¬ 
nately light and dark, the periods becoming longer and longer 
as the synchronism becomes more perfect. 

17. The process of starting an alternator and connecting 
it in parallel with one or more others already in operation is 
as follows: Suppose machine A, Fig. 8, to be running at full 
speed and voltage. If machine B is now started and the plug 
is inserted at m, the lamps will at first rapidly fluctuate in 
brightness, but as B approaches synchronism with A, the 
fluctuations will become slower. When they have become 
as low as one in 2 or 3 seconds, the main switch M' is thrown 
in at the middle of one of the beats when the lamps are dark. 
In some cases, the connections are so made that the lamps 
are bright when synchronism is attained. It is evident that 
this could be done by reversing the connections of one of the 
transformers. Whether the state of synchronism will be 
indicated by light or dark lamps depends simply on whether 
the transformer secondaries are connected so as to assist or 
to oppose each other. 

18. Synchronizing' Two-Phase and Three-Phase 
Machines.— If one phase of a two-phase or of a three-phase 
alternator is in synchronism with a corresponding phase of 
another alternator, the other phases will be in synchronism 
—provided, of course, that the machines are properly con¬ 
nected. Synchronizing circuits are, therefore, connected to 
only one phase of such alternators. But to insure that the 
connections are correct it is well temporarily to connect a pair 
of transformers across the other phases. For example, on a 
two-phase machine, an arrangement similar to that shown in 
Fig. 8 should be made for each of the phases, and when the 
connections are right and the machines are in phase, each 
set of phase lamps will be dark or light, as the case may be, 
at the same instant, showing that both phases are ready for 
parallel operation. After it is known that the connections 
are all right, the temporary pair of transformers may be 
removed and only one pair used. 


DYNAMO-ELECTRIC MACHINERY, PART 2 73 


19. Fig. 9 shows a common scheme of connections used 
for synchronizing three-phase alternators with lamps. In 
this case, the connections are shown for three machines, 
each machine being provided with its plug receptacle p. 
The primary coil of one small transformer t is connected 
across two phases of the main bus-bars, and that of the other t' 
across the synchronizing bus-bars, through which connection 
can be made to the same phase of any one of the machines 
by inserting a plug in the proper receptacle. For example, 



suppose that the main switch of machine 1 is closed, as indi¬ 
cated by the dotted lines, and that it is desired to operate 
machine 2 in parallel with 1 . Machine 2 should first be 
brought up to speed and a plug inserted in receptacle 2, 
thus connecting t f to the machine. Synchronism is here 
indicated when the lamps burn to full brightness; hence, the 
generator switch of machine 2 will be thrown in when the 
lamps are at the middle of a beat and at full brightness. 
The same arrangement can be used for synchronizing with 































































74 


OPERATION OF 


dark lamps, the only change being that the synchronizing 
plug then will be cross-connected, thus making the trans¬ 
formers oppose each other. With alternators built to gen¬ 
erate a low voltage, as is sometimes the case when they are 
used in connection with step-up transformers or for low- 
voltage work, it is not necessary to use transformers /, t'\ 
the terminals of the synchronizing circuit may be connected 
directly to the machines or bus-bars and a sufficient number 
of lamps used in series to stand the maximum voltage 
applied to them. 

20. Synchronizing Instruments.—The use of lamps, 
as already explained, tor indicating synchronism has been 
very common; but for large units this plan is not entirely 
satisfactory, because they do not indicate slight phase differ¬ 
ences, which may cause large cross-currents to flow when the 
machines are switched together. A voltmeter is sometimes 
used in place of the lamps and the connections are so made 
that when the machines are approaching synchronism the 
voltmeter needle is near the middle of the scale, where it is 
very sensitive to slight differences in phase of the alter¬ 
nators. Synchronism indicators, synchronoscopes, and vari¬ 
ous other devices are also in use. There are many possible 
arrangements and modifications of connections, but the prin¬ 
ciples involved are the same in all and the object in all cases 
is to avoid throwing the machines together at the wrong 
time. 


21. Alternators running in parallel will hold each other 
in step, and in doing this local or cross-currents may flow 
from one machine to the other. The division of the load 
cannot be regulated by adjusting the field excitation, as in 
the case of direct-current dynamos in parallel, but must be 
made by adjusting the engine governor. Adjusting the field 
excitation of alternators results only in changing the amount 
of local current flowing between the two machines and this 
should be made as small as possible; that is, the sum of the 
currents delivered by all the alternators should be made as 
nearly as possible equal to the total current supplied to the 


DYNAMO-ELECTRIC MACHINERY, PART 2 75 


line. The output of each machine will depend on the energy 
supplied to it by the engine. The distribution of the kilowatt 
load among the alternators is dependent on the relative phase 
positions of their rotating parts. Alternators in parallel 
must run in synchronism, yet it is possible by increasing the 
energy supplied to the prime mover of one alternator to force 
temporarily the rotating parts of that alternator ahead of the 
others in relative phase position. The alternator will then 
take on a greater share of the total load. Prime movers with 
governors that may be adjusted while the machines are run¬ 
ning are used for the control of the load division between 
alternators in parallel. Setting the governor so that the 
prime mover takes more energy causes its alternator to pull 
ahead in phase and increases its load. Adjusting the gov¬ 
ernor so that the prime mover takes less energy causes its 
alternator to lag behind in phase and decreases its load. 
When cutting out an alternator, the governor is adjusted to 
decrease the load and then the switch is opened. 

22. What has been said regarding the synchronizing of 
alternators applies also to synchronous motors and rotary 
converters; each must be synchronized before being con¬ 
nected with the alternating-current circuit. 

23. Hunting of Alternators.— Alternators in parallel 
frequently give trouble from what has been termed surging 
or hunting; that is, the speed may vary periodically during 
certain portions of each revolution causing momentary cross¬ 
currents to flow. These currents may become so large as 
to seriously interfere with the voltage of the system. Alter¬ 
nators driven by large slowly moving reciprocating engines 
most often give trouble from this cause; the engine crank 
receives a certain number of impulses during each revolution 
and at each of these the angular velocity is increased a trifle. 
If the alternator to which the engine is connected has a 
large number of poles, a very slight change in angulaf 
velocity will produce a considerable phase difference. Alter¬ 
nators driven by steam turbines are less likely to hunt, 
because the angular velocity is more uniform. 


76 


OPERATION OF 


Various devices have been used to overcome hunting, 
including improvements in engine governors, the use of 
heavy flywheels, the use of multiple-expansion engines, etc. 
Special windings or short-circuited conductors are some¬ 
times used on the alternator pole faces so that suddenly 
shifting field magnetism caused by the surging current will 
induce currents that tend to retard or dampen such changes. 


SWITCHBOARD APPLIANCES 


TTEIiD RHEOSTATS 

24, In order that the transmission system shall be under 
control and also that the condition of the lines, the amount 
of output, etc. shall be known, it is necessary to have vari¬ 
ous controlling, protective, and measuring devices in the 
station. These consist of field rheostats; switches; fuses and 
circuit-breakers; ground detectors; lightning arresters; measur¬ 
ing instruments; including voltmeters, ammeters, wattmeters, 
etc., and other auxiliary devices. 

25. Little need be said regarding field rheostats in 
addition to what has already been given. They consist of a 
resistance so arranged that it can be cut in or out of a cir¬ 
cuit by steps. The resistance material may consist of 
German-silver or iron wire, or sometimes of cast grids. 
Wire or strip resistance is usually wound or assembled on an 
insulating base of some sort and afterwards covered with an 
insulating and heat-conducting material. The total resist¬ 
ance should be about the same as that of the field to be 
controlled. 

Unless the rheostat is very large, it is mounted either on 
the front or on the back of the switchboard. Very large 
rheostats are sometimes mounted at some convenient place 
some distance from the switchboard and are operated by 
small motors that are controlled from the switchboard. 
In any case, the rheostat handle is placed on the front of the 
board within easy reach of the attendant. 




DYNAMO-ELECTRIC MACHINERY, PART 2 77 


SWITCHES 


XiOW-TENSION SWITCHES 


26. Probably the most important appliances on the switch¬ 
board are the switches. These must have ample carrying 
capacity and be capable of breaking the full-load current of 

the generator or circuit, with¬ 
out destructive burning or 
arcing. The style of switch 
used for any installation will 
depend on the voltage and 
current to be handled. 

Plain knife switches are 



Fig. 11 



generally used for pressures up to 1,000 volts, and this style 
of switch with a broad separation of the blades and contacts 
has been used on pressures as high as 2,500 volts. For work 
of the latter class, however, it is preferable to use a switch 
of the quick-break variety (Art. 27), and even for pressures 
as low as 500 volts, quick-break knife switches are often 
used. Fig. 10 shows a typical two-pole knife switch designed 
for front connections and provided with fuses. Fig. 11 shows 
a similar switch without fuses and intended for mounting on a 


444—6 


























78 


OPERATION OF 


switchboard. When the switch is opened, connection is broken 
between the two clips, on each side respectively, thus opening 
both sides of the circuit simultaneously. A knife switch should 
be mounted with the handle up when the switch is closed, so 
that, when open, the switch will not tend to fall closed. 

27. Quick-Break Switches.—Fig. 12 shows a style of 
quick-break switch that has proved very successful and 
is suitable for pressures as high as 2,000 to 2,500 volts if the 
current is not large. The switch blade, of drawn copper, is 




made in halves, a, b, which are connected by two springs c, one 
on each side of the blade. When the handle is pulled out, the 
half a leaves the clip d and thus stretches the springs. When 
the bottom blade flies out, it leaves clip d very quickly, thus 
drawing out the arc and breaking it almost instantaneously. 


HIGH-TENSIOX SWITCHES 

28. Knife-blade switches of the types used for direct 
current are, without modification, suitable for use on low- 
tension alternating-current circuits. The current-carrying 































































DYNAMO-ELECTRIC MACHINERY, PART 2 79 


capacities of small knife-blade switches are the same for alter¬ 
nating current as for direct current. In the medium and large 
sizes, however, the switches are likely to be heated more by an 
alternating current than by a direct current of equal amperage. 

29. Oil Switches.—Knife-blade switches or air-break 
circuit-breakers are not suitable for use in high-voltage circuits 
on account of the long and dangerous arcs that are produced 



when circuits carrying currents at high potential are broken in 
air. For such work oil switches are used. These are so 
designed that the point at which the electric circuit is made or 
broken is situated under the surface of a high-resistance oil 
contained in a closed vessel. The weight of the oil and its 
cooling effect combine to smother the arc formed, breaking the 
circuit, and the length of the arc is reduced to only a fraction 
of what it would be if the opening were in air. The oil wells 
are usually made of cast iron or sheet iron lined with wood. 


























































80 


OPERATION OF 


30. On potentials of 4,000 volts or over, it is common prac¬ 
tice to open each line of a circuit in a separate oil vessel, or in 
a separate compartment of one vessel, and to open each line 
simultaneously in two places, by arranging two sets of contacts 
in series. Fig. 14 shows a 15,000-volt, 300-ampere, two-phase, 
oil switch with the oil well removed. Leads are brought to each 
pair of fixed contacts a through porcelain bushings b set into the 
cast-iron cover on the under side of which the oil well is sup¬ 
ported. Each movable contact consists of two parts, a main 

contact c of heavy 
sheet-copper brushes 
and a pair of auxiliary 
contacts d consisting 
of short, movable, 
copper cylinders held 
in place by helical 
springs. On opening 
the switch, the final 
break is at the auxil¬ 
iary contacts, which 
take the arc. The 
movable contacts are 
carried on wooden 
rods, which are lifted 
by operating a system 
of levers known as a 
link mechanism. In 
Fig. 14 the switch is shown in the open position. 

Oil switches are operated manually, electrically, or pneu¬ 
matically. Manually operated oil switches are suitable for use 
on circuits carrying moderate amounts of energy. They are 
operated by a handle connecting through a link mechanism to 
the vessel carrying the moving contacts. For switches mounted 
on the backs of the switchboard panels the linkage is very short. 
When it is preferred to place the switches farther away, the 
links are made longer and the direction of motion is changed 
by means of bell-cranks, as shown in Fig. 15, which shows oil 
switches operated by hand. 




































DYNAMO-ELECTRIC MACHINERY, PART 2 81 


FUSES AND CIRCUIT-BREAKERS 

31. Either fuses or circuit-breakers may be used to pro¬ 
tect the generators or circuits from an excessive flow of 
current, due either to a short circuit or to an overload. Fuses 
are not used as much as they once were, as it has been found 
that circuit-breakers are more reliable. The circuit-breaker 
may be a separate device, or the main switch may be pro¬ 
vided with an automatic tripping device that will open the 
switch when the current exceeds a given amount. 


FUSES 

32. A fuse consists of a strip or wire of fusible metal 
inserted in the circuit and so proportioned that it will melt 
and open the circuit if the current for any reason becomes 
excessive. Fuses are often made of a mixture of lead and 
bismuth, though copper and aluminum are also used. Alu¬ 
minum is used very largely for high-tension fuses. 

For low-tension switchboards, plain open fuses may be 
used; but for high-tension work it is necessary to have them 
arranged so that the arc formed when they blow will not 
hold over. Moreover, it is necessary to have high-tension 
fuses arranged so that they can be renewed without danger 
to the switchboard attendant. 

Fuses are still much used on alternating-current boards 
and also for protecting individual parts of direct-current 
circuits. The trouble and delay caused by the frequent 
blowing of fuses sometimes cause attendants to use a 
heavier fuse or a piece of copper wire, thus removing all 
the protection for which the fuse was installed. A fuse 
should be selected of such size that it will blow before any 
part of the circuit that it is designed to protect can be injured. 
If it be found that the proper size of fuse blows repeatedly, 
the circuit should be examined to find the cause of the trouble. 

33. Enclosed Fuses.—Many types of enclosed fuses 
are available and some form of these should be used wher¬ 
ever there is any possible danger of fire. 



82 


OPERATION OF 


34. Fig. 16 ( a) shows a type of fuse block, half in 
section, used by the General Electric Company on alter¬ 
nating-current switchboards; (b) shows the shape of the 
aluminum fuse used in the block. The fuse holder is made 
in two parts, the lower part A being of porcelain and the 
upper part B of lignum vitae. The lower part is provided 
with blades c that fit between the clips d , d' in the same way 
as the blades of a knife switch. These clips lie in slots in 
the marble board F and are connected to the line and 
generator by means of terminals g and h. By adopting this 



(b) 


Fig. 16 

arrangement, the whole block may be detached from the 
board by simply pulling it straight out, thus pulling the 
blades out of the clips. The fuse is shown at /, and is 
clamped by means of the screws m,n. A vent hole / is 
provided in the lignum-vitae cover, and the rush of air 
through this vent, together with the confined space, results 
in the suppression of the arc. This fuse block is suitable 
for currents up to 150 amperes at 2,500 volts. For higher 
pressures, fuse blocks are used in which the fuse is pulled 
wide apart as soon as it blows, thus breaking the arc. 
































DYNAMO-ELECTRIC MACHINERY, PART 2 83 


CIRCUIT-BREAKERS 

35. A circuit-breaker is essentially an automatic 
switch that opens the circuit whenever the current exceeds 
the allowable limit. It is therefore intended more as an 
automatic safety device than a switch for regularly opening 
or closing the circuit. 

Circuit-breakers are made in great variety to handle 
currents varying from a few amperes up to several thousand; 
they are constructed for both alternating and direct currents. 
In nearly every case, they consist of a switch of some kind 
that is held closed against the action of a spring. The main 
current passes through an electromagnet or solenoid, and 
when the current for which the breaker is set is exceeded, 
this magnet attracts an armature or core and operates a trip 
thus allowing the 
switch to fly out. In 
some cases, the 
breaker opens both 
sides of the line, 
though often it is 
single-pole and opens 
one side only. A 
single example will 
show the general 
method of operation. 

30. In the single¬ 
pole, direct-current, 
overload circuit- 
breaker, Fig. IT, 
the main contact a is 
laminated and is 
pressed against the 
contact surfaces by 
means of the handle working through a togglejoint at c. The 
tripping coil is shown at d ; and when the current exceeds the 
amount for which the breaker is set, the core inside d is 
suddenly drawn up, this striking a trigger and allowing the 


























84 


OPERATION OF 


breaker to fly out. The position of the core in d can be 
changed by adjusting screw e, thereby varying the current at 
which the breaker trips. Auxiliary carbon contacts b, b do 
not open until after the main contact, so that the burning action 
is confined to the carbon contact surfaces. 

Many overload circuit-breakers are very similar in 
general appearance and operation to the type shown in 
Fig. 17, the main difference being in the arrangement of the 
tripping coil. 

It has been suggested that for use in gaseous mines, 
switches, fuses, circuit-breakers, etc. be provided with gauze 
covers; this has been tried in some European mines, but it 
is not done in the United States. 


GROUND DETECTORS 

37. Ground detectors are used to determine whether 
or not a transmission line that should normally be insulated, 
is in contact with the ground or any conductor leading to the 
ground. A voltmeter makes a very good ground detector, 
because it not only indicates whether a ground is present, 
but by its deflection it shows whether the path of the current 
to ground is one of high or low resistance. Most ground 
detectors have a permanent ground, as indicated by G in 
Figs. 18, 19, and 20. 

In order to indicate grounds, the voltmeter may be con¬ 
nected as shown in Fig. 18 (a). If the line a is grounded 
at G' } as indicated by the dotted line, no deflection will result 
when the switch blade c is placed on point 1. If, however, 
the blade is moved to point 2, current will pass from 
line a through the ground on the line, and the permanent 
ground G of the detector, to the voltmeter, to point 2, and 
thence to the line b, thus completing the circuit. When a 
deflection is obtained on point 2, it shows that line a 
is grounded; and when obtained on point 1 it shows that 
line b is grounded. Fig. 18 ( b ) shows an arrangement for 
connecting the ordinary voltmeter so that it may be used both 
to measure the line pressure and as a ground detector. When 



DYNAMO-ELECTRIC MACHINERY, PART 2 85 


the switch is in the position 1-1', the voltmeter V is con¬ 
nected directly across the line and gives the voltage on the 
system; when in the position 3-3', V indicates any grounds, 
such as G", that may be present on line b; when in the 
position 2-2', V indicates grounds on line a, as at G'. It is 



evident that the needle will swing in the opposite direction 
for a ground on a than for one on b and also that the degree 
of the deflection will indicate whether the ground is of high 
or low resistance. 





















86 


OPERATION OF 


38. Another common arrangement for detecting grounds 
is shown in Fig. 19, where two incandescent lamps c,d are 

connected in series across 
the lines. The voltage for 
which these lamps are de¬ 
signed is equal to that of 
the dynamo, so that when 
the two are connected in 
series, they will burn dull 
red. From a point between 
the lamps, a connection is 
made to ground through a 
switch or push button /. If contact is made at / and there is 
no ground on either line, the brilliancy of the lamps will not 
be altered. If there is a ground on b, as indicated at G ', 
lamp c will burn brighter than d ; or if the resistance of the 
ground connection is low, c will burn at full brilliancy and d 
will become dark. 



Fig. 19 


39. The ground detectors just described apply more 
particularly to low-tension direct-current installations, but 



similar arrangements may be adapted to high-tension, 
alternating-current systems by using potential transformers. 
Fig. 20 shows one method that has been used in some 
cases on alternating-current switchboards. The regular 
voltmeter V with which the switchboard is equipped 






















DYNAMO-ELECTRIC MACHINERY, PART 2 87 


is here used also as a ground detector. P is a plug switch 
by means of which points 1 and 2 or 1 and 3 may be 
connected together. Under ordinary conditions, the plug is 
in 1 and 2, thus connecting the primary of the potential 
transformer across the line, and V serves as an ordinary 
voltmeter. 5 is a key normally resting against 4 , but which 
may be depressed so as to connect one side of the line to 
ground through the transformer primary. If there happens 
to be a ground on the side b , as shown at G ', the voltmeter 
will give a reading when 5 is pressed. By placing the plug 
in points 1 and «?, side a may be tested in like manner for 
grounds. When the key 6* is not pressed, V is connected as 
an ordinary voltmeter. 

40. El ectrostatic Ground Detectors. —Ground 
detectors operating on the electrostatic principle are much 
used on high-pressure alternating-current switchboards. 
They have the advantage that they require no current for 
their operation and hence may be left connected to the 
circuit all the time, thus indicating a ground as soon as it 
occurs. They also give an indication without its being 
necessary to make an actual connection between the line 
and ground as is the case with all the detectors previously 
described. Fig. 21 (a) and (b) } respectively, shows the 
general appearance and illustrates the principle of a Stanley 
electrostatic ground detector especially adapted to high- 
pressure, alternating-current lines because the instrument is 
not in actual connection with either of the lines. The fixed 
vanes 1 and 4, 2 and 3 are connected together in pairs, as 
shown, the two pairs being connected respectively to 
plates <z', a of two small condensers b , b\ which consist simply 
of two brass plates, mounted in hard rubber but separated 
from each other. Plates b , b f of the condensers are con¬ 
nected to the lines A , B. The movable vane V is connected 
to the ground and is held in the central position shown in the 
figure by means of small spiral springs S. To explain the 
operation, consider an instant when line B is positive. Then 
plate b' will be positive and a negative charge will be 


88 


OPERATION OP 


induced on plate a\ repelling an equal positive charge to 
plates J and 4. At the same instant, line A and plate b will 



i 

ib) 

Fig. 21 

be negative, plate a positive, and plates 2 and 3 negative. 
If, at that instant, line B is grounded as shown at G ", con¬ 
nection through ground from G" to G and thus to movable 

















DYNAMO-ELECTRIC MACHINERY, PART 2 89 


vane V will make the potential of V positive, or the same 
as plates 1 and 4, and V will therefore be repelled by plates 1 
and 4, and attracted by plates 2 and 3 because these two 
plates are negative. If, instead of a ground on B, line A is 
grounded as shown at G' } V will be of the same potential as 
2 and 3 and will be repelled by them and attracted by 1 and 4. 
With B grounded, then, the pointer will swing to the right, 
and with A grounded, to the left. 

Instruments of this kind can, of course, only be used in 
places where the pressure is fairly high, as the electrostatic 
forces produced by charges due to low pressures will not be 
large enough to operate an instrument unless it is made much 
too delicate to be of practical use in a light or power station. 
In most electrostatic detectors, the lines are connected directly 
to the fixed sectors 4, 2, 3, 4 and the condensers C, C’ are 
omitted. 


PROTECTION FROM LIGHTNING AND STATIC 
CHARGES 

41. Sources of danger to electrical equipments may arise 
outside the station and may cause great loss unless ample 
provision is made for protection. Among these may be men¬ 
tioned danger from lightning, danger from static charges, or 
other effects commonly referred to as static , and danger from 
short circuits caused by either of the former. Damage from 
lightning occurs on systems having overhead lines, but static 
charges and the damage resulting therefrom can occur on 
systems having either overhead or underground lines. 


PROTECTION FROM LIGHTNING 

42. Lightning Arresters. —Differences of potential 
often arise between the atmosphere and the earth. These 
differences cause discharges of atmospheric electricity that, 
in seeking a path of low resistance to earth, frequently follow 
overhead electrical conductors into switchboards or dynamos 
where, unless proper precautions are taken, great damage 
may result. These precautions consist of furnishing a path 




90 


OPERATION OF 


easier for the discharge to follow on its way to earth, than 
to go through the insulation of a machine or to arc across 
terminals on a switchboard. A device for diverting lightning 
discharges away from an electric circuit and guiding them 
to earth is called a lightning arrester. 

43. A simple lightning arrester is shown diagrammatically 
in Fig. 22. A lightning discharge is generally oscillatory in 
character; that is, it alternates in direction, but the potential 
is greatest at the beginning and decreases gradually to zero. 
It will not pass through an inductive path if an alternative 
non-inductive path is provided for it. The choke coils A , A , 
therefore, act as an obstruction to the discharge, which pre¬ 
fers to jump across the air gaps g,g between plates 1,3 



and 2, 3 and thence to ground. The air gaps g, g must be 
long enough so that the generator pressure will not cause the 
formation of arcs. A distance of ^ inch should be sufficient 
for pressures up to 500 volts. 

44. Suppression of Arcing.—With connections as 
shown in Fig. 22, a discharge from both lines at the same 
time should cause the formation of arcs across each gap. 
The current from the generator would follow these arcs and 
the machines would thereby be short-circuited. This large 
short-circuited current would melt the plates of the arrester 
and might damage the machine. The arc must, therefore, be 
suppressed as soon as the discharge has passed and the 
arrester must be left in condition for the next discharge. 















DYNAMO-ELECTRIC MACHINERY, PART 2 91 


This may be accomplished either by an automatic device that 
will lengthen the gap until the arc is broken; or by a mag¬ 
netic field arranged to blow out the arc; or by causing the 
arc to be formed within a confined space so that it will be 
smothered; or by making the plates or terminals, between 
which the arc forms, of a non-arcing metal, the vapor of 
which forms a high resistance path. 

45. Location of Lightning Arresters.—Lightning 
arresters should be placed not only at the power house, but 
at each place where discharges may work damage by passing 
to ground through insulation or apparatus. On long trans¬ 
mission lines, arresters are 
sometimes mounted on 
poles at regular intervals. 

Sometimes a separate 
barbed iron wire is strung 
along the tops of the poles 
and connected at frequent 
intervals to the ground. 

By this’ method, the dis¬ 
charges are carried to earth 
continuously and thus pre¬ 
vented from reaching a 
potential sufficiently high 
to work injury. 

46. Connections to 
Ground.—Unless the 
ground connections are 
good, arresters will be use¬ 
less. The Westinghouse 
Company recommends a 
ground connection shown 
in Fig. 23. A galvanized- 
iron pipe is driven well into 
the ground and the top of it surrounded by coke, which retains 
moisture; the wire is run down the pole and connected to the 
top of the pipe as indicated. The wire is sometimes incased 



Fig. 23 










92 


OPERATION OF 


in galvanized-iron pipe for about 6 feet from the base of the 
pole, and if this is done it is well to solder the ground wire 
to the top of the pipe at a. The following method of making 
the ground connections at the station is recommended: A 
hole is dug 6 feet square and 5 or 6 feet deep in a location 
as near the arresters as possible, preferably directly under 
them. The bottom of this hole is then covered to a depth of 
about 2 feet with charcoal or coke crushed to about pea size. 
On top of this is laid a tinned, copper sheet, about 5 feet 
square, with the ground wire (about No. 0 B. & S.) soldered 
completely across it. The plate is then covered with a 2-foot 
layer of coke or charcoal and the remainder of the hole filled 
with earth, running water being used to settle it. This 
will give a good ground, if made in good rich soil; it will 
not give a good ground in rock, sand, or gravel. Sometimes 
grounds are made by putting the ground plate in a running 
stream. This, however, does not give as good a ground as 
is commonly supposed, because running water is not a par¬ 
ticularly good conductor and the beds of streams very often 
consist of rock. When lightning arresters are installed, all 
wires leading to and from them should be as straight as pos¬ 
sible. Bends act more or less like a choke coil and tend to 
keep the discharge from passing off by way of the arrester. 

47. Selection of Lightning Arresters.—Some arrest¬ 
ers will work on either direct- or alternating-current circuits 
but, generally speaking, the arrester should be selected with 
reference to both the voltage of the circuit and the kind of 
current. The variety of arresters adapted both to direct 
and to alternating currents is too great to attempt their 
description here. _ 


STATIC CIIARGES 

48. High-pressure systems are sometimes subjected to 
pressures very much higher than the normal by what are 
known as static charges. Any sudden change in the 
electromotive force is likely to cause these; as, for example, 
switching a high electromotive force on to a circuit, switch¬ 
ing a transformer into circuit, etc. This effect is somewhat 



DYNAMO-ELECTRIC MACHINERY, PART 2 93 


similar to that caused by suddenly checking the flow of 
water in a pipe. If a valve be suddenly closed, the impetus 
of the water flowing in the pipe will cause the pressure to 
rise much above the normal, producing the well-known 
water-hammer effect. To guard against breaking down 
insulation by high static charges, devices very similar to 
lightning arresters are used. In fact, a number of large 
plants have their lines fully equipped with lightning arresters, 
even though the distributing lines are entirely underground 
and hence safe from lightning discharges. The lightning 
arresters are in such cases installed to protect the cables 
against abnormal pressures caused by the so-called static 
effects. 


MEASURING INSTRUMENTS 

49. Instruments for measuring electrical quantities are 

made in many forms and varieties. They all depend on 
reactions or heating effects caused by the passage of an 
electric current through some portion of the instrument. 
Commercial instruments may now be had, on which may be 
read at a glance and with great accuracy the electromotive 
force of a circuit in volts, the current in amperes, the 
energy in watts, etc. _ 

VOLTMETERS AND AMMETERS 

50. Among the best commercial voltmeters and 
ammeters are those in which the movement of the needle 
depends on the reaction between a fixed permanent magnet 
and a movable coil through which a current is caused to 
flow. Such a portable voltmeter and also an ammeter 
have already been described in Elements of Electricity and 
Magnetism. The Thompson inclined coil instruments depend 
for their action on the movement of an iron vane in a magnetic 
field set up by the passing current. Other instruments employ 
the principle of the varying length of a conductor as it is 
heated by a passing current. 

51. These instruments are made either to indicate the 
values of the quantities to be measured or to record the values 




94 


OPERATION OF 


on a moving paper dial. The two kinds are distinguished 
by the terms indicating instruments and recording instruments. 
Unless the current to be measured is very small, only a 
portion of it is allowed to flow through the ammeter. A 
resistance called an ammeter shunt is arranged to carry 
the larger part of the current, but as the portion through the 
instrument is always proportional to the total current, the 
ammeter may be calibrated to read the total amperes in 
the circuit. Similarly, when a very high pressure is to be 
measured, a known resistance is generally used in series 
with the voltmeter to keep the current down to the capacity 
of the instrument. 


WATTMETERS 

52. On direct-current circuits, the power, in watts, being 
used at any instant may be found by taking the product of 
the volts and amperes. Instruments known as indicating 
wattmeters automatically perform this multiplication and indi¬ 
cate the watts passing at any time. Other instruments 
known as recording wattmeters perform the same multiplica¬ 
tion and also introduce the element of time; that is, they 
record the number of watt-hours , or kilowatt-hours , that have 
passed during a given time. 

Fig. 24 (a) shows an assembled Thompson recording 
wattmeter with the cover removed; view ( b ) shows the 
rotating part, or armature removed from the meter. Fixed 
coils a , a , called field coils, are connected in series with one 
side of the circuit. Movable coils b , b, b , called armature 
coils, are wound across a suitable support, and are connected 
through commutator c, c y in series with a resistance and a 
so-called shunt coil g across the main circuit. Coil g consists 
of a number of turns of fine wire and is mounted on an adjust¬ 
able brass frame h so that the coil can be moved in or out, 
that is, to or from the armature, so as to compensate for 
friction on light loads; the coils provide a magnetic field 
almost sufficient to move the armature when no current is 
flowing in the series-coils; hence a small load starts the 
meter. The moving element, or armature, is very similar 



DYNAMO-ELECTRIC MACHINERY, PART 2 95 


to a generator or motor armature except that no iron is used 
in the core. Shaft d has a hardened-steel pivot n at the 
lower end and a worm-gear m at the upper end. The position 
of the armature shaft can be seen in Fig. 24 ( a ); the pivot n 
rests on a jewel and the worm-gear w, as the armature turns, 
actuates a train of gears, that moves the hands on the dials. 
Current is conveyed to the movable coils through brushes /, /. 



Fig. 24 


A disk k , formerly made of copper but now made of alumi¬ 
num, is connected to shaft d> which causes it to rotate 
between the poles of permanent magnets e.e.e. 

53. Operation of a Thompson Recording Watt¬ 
meter.—As the disk k rotates, there are set up in it eddy 
currents that are directly proportional to the speed of rota¬ 
tion and that cause a retarding or damping effect. The torque 
of the armature, and consequently the speed, is proportional 
both to the amount of current through the fixed coils, or field 
coils, a , a y and the current through the movable, or armature, 
















































96 


OPERATION OF 


coils by by b . The current through a , a is the current in the cir¬ 
cuit being tested, or proportional to it, and that through by b is 
proportional to the pressure of the circuit; hence the speed of 
the armature is directly proportional to the watts consumed 
in the circuit. This instrument is really a small electric 
motor with no iron in the magnetic circuit. Iron is not used, 
because its permeability is variable, depending on the degree 
of saturation. This meter will operate on either direct- or 
alternating-current circuits and will give accurate results if 
the commutator, pivot, and jewel are kept in good condition. 

There are a number of other meters in common use, most 
of which are of the motor form. Some are made for use on 
alternating-current circuits only. Other instruments are 
sometimes used for making measurements of electric quan¬ 
tities, but those described are most commonly used around 
central stations. 


SWITCHBOARDS 

54. The switchboard is a necessary part of every plant. 
Its object is to group together at some convenient and acces¬ 
sible point, the apparatus for controlling and distributing the 
current, and the safety devices for properly protecting the lines 
and machines. Scarcely any two switchboards are alike in 
every particular; their layout and the type of apparatus used on 
them depend on the character of the system used, the num¬ 
ber and size of generators, the number of circuits supplied, etc. 

55. Construction. —Switchboards are now usually made 
of slate, marble, soapstone, or brick tile. Occasionally, where 
cheaper construction is required, a skeleton framework of 
seasoned hardwood is used, the wood being filled and var¬ 
nished to prevent absorption of moisture. If connections are 
to be made on the back of the board, ample room should be 
left between the board and the wall, so that the work can be 
done without danger or discomfort. Switchboards are now 
usually built in panels, those carrying instruments for gen¬ 
erators being known as generator panels, and those carry¬ 
ing instruments for feeder circuits, as feeder panels. 



DYNAMO-ELECTRIC MACHINERY, PART 2 97 


DIRECT-CURRENT SWITCHBOARDS 


56. Railway Switchboard.—Fig 1 . 25 shows a typical 
direct-current switchboard as arranged for electric rail¬ 
way operation on the ordinary 500-volt rail-return system. 



The board consists of three generator panels A , A> A , one 
total-output panel B , and five feeder panels C y C, etc. One 
of the generator panels is left blank to provide for a future 
generator. Each generator panel is equipped with + and — 
main switches l y l y voltmeter plug 2, field switch 3 for open¬ 
ing the field circuit of a generator and at the same time 






































98 


OPERATION OF 


closing a path for the field to discharge through a resistance 
pilot-lamp receptacle 4, field rheostat operated by hand 
wheel 5, ammeter 6, circuit-breaker 7, and station lighting 
switch 8. The total-output panel carries a voltmeter 9 that 
can be connected to either machine by means of the volt¬ 
meter plug, a total-output ammeter 10 that indicates the 
combined current output of the generators; and recording 
wattmeter 11 that records the total output in kilowatt-hours. 
Each feeder panel is equipped with a single-pole feeder 
switch 12, a feeder ammeter 13, and a feeder circuit- 
breaker 14. Since on a ground-return railway system the 
current returns through the rails, which are connected to 
the negative bus-bar, the feeders are connected to the posi¬ 
tive bus-bar only, hence single-pole feeder switches are used. 

Fig. 26 shows the connections for the board. Two feeder 
panels only are shown, and the instruments and switches are 
numbered to correspond with Fig. 25. If lightning-arrester 
reactance coils are used on the switchboard, they will be 
inserted as indicated on the left-hand feeder panel. The equal¬ 
izer switches are mounted on pedestals near the generators 
and the equalizer connections are not brought to the switch¬ 
board. When the voltmeter plug is inserted in either recep¬ 
tacle, terminals a, c and b, d are connected, thus placing the 
voltmeter across either machine; the voltmeter connections are 
made at the lower terminals of the main switch, or “back” of 
the switch, so that voltmeter readings can be taken before a 
machine is thrown in parallel by closing the switch. 


ALTERNATING-CURRENT SWITCHBOARDS 
57. The arrangement of ordinary alternating-current 
boards is, in many respects, similar to that of direct-current 
boards. They are usually built up in panels in the same way 
as the boards previously described. Owing to the fact that 
alternators are generally separately excited, the switchboard 
contains some extra apparatus connected with the exciter 
that is not found on direct-current boards. The wiring and 
connections will also depend on whether single-phase or 
polyphase alternators are used. 



















































































































































































































































































DYNAMO-ELECTRIC MACHINERY, PART 2 99 


58, General Arrangement of High-Pressure 
Switchboards. —In low-pressure work, the switchboard 
consists of a group of slate or marble panels on which the 
switches, bus-bars, instruments, and all devices necessary 
for the control of the station output are placed. Such crowd¬ 
ing of the parts is dangerous on a high-pressure board, and 
the tendency in large stations is to separate the high-pressure 
switches and bus-bars so that a short circuit on one part will 
not spread to others and result in a serious interruption of the 
service. The switchboard panels in this case carry only the 
instruments and small switches necessary for controlling the 
main switches that are usually operated either by compressed 
air, electric motors, or electromagnets. No parts carrying 
high pressure are exposed on the surface of the board, thus 
insuring safety to the attendant; a switchboard arranged on 
this plan occupies a large amount of space. 


PERSONAL SAFETY FROM ELECTRICAL SHOCKS 

59. It would be perfectly safe to handle high-pressure 
electrical conductors if, while doing so, no part of the body 
should form an electrical connection between points at widely 
different potentials. If a high-pressure generator is grounded 
either accidentally or on purpose, an electrical connection 
made through the body between any part of the generator, or 
any conductor connected thereto, and the ground, may cause 
a shock. It is therefore best, when handling machines or 
conductors on which there is a pressure of 500 volts or more, 
to use rubber gloves, or rubber shoes; even to stand on a 
piece of dry wood is sometimes sufficient. Tools, screw¬ 
drivers, pliers, wrenches, etc. with insulated handles are 
very convenient for such work. Even when standing on an 
insulator, never let but one bare hand come in contact with 
a live conductor at the same time. It is still safer to avoid 
handling high-pressure conductors as much as possible. 
The adage “Familiarity breeds contempt” applies with full 
force to electrical workmen, for those accidentally killed are 
nearly always the men who from long experience have 
become careless. 



100 


OPERATION OF 


STORAGE BATTERIES 


INTRODUCTION 

GO. Comparison Between Primary and Secondary 
Cells. —A primary cell consists of two unlike electrodes 
immersed in an electrolyte, whereby an electromotive force is 
developed between the electrodes, and an electric current is 
set up when the terminals of the electrodes are connected to 
an electric circuit. The direction of the current is from the 
positive terminal to the negative terminal. The flow of elec¬ 
tricity is accompanied by chemical changes on the surface of 
at least one of the electrodes and, usually, of the electrolyte, 
as a whole. The quantity of the material altered by these 
chemical changes is proportional to the quantity of electricity, 
in ampere-hours, that flows through the circuit. When any of 
the materials entering into the chemical changes of the primary 
cell has been entirely altered, the cell is exhausted, or fully 
discharged . 

Gl. The action of a secondary cell, storage cell, or 
accumulator, is fundamentally the same as that of a primary 
cell, but differs in this that when the secondary cell is dis¬ 
charged, either wholly or partly, the chemical action may be 
reversed and the storage cell restored to its original state. 
This reverse action, known as charging, is caused by passing 
a current through the cell in the reverse direction; that is, by 
letting the current enter at the positive terminal. The material 
of the electrodes that undergoes chemical changes during 
charge and discharge, called the active material, is generally 
supported on the surface or in openings, or pockets, of the 
electrode, which is then called a grid. The grid with its active 
material is called a plate. Each electrode in a storage c^U con- 



DYNAMO-ELECTRIC MACHINERY, PART 2 101 


sists of one plate or of several plates connected in parallel. 
There is sufficient space between the plates of both electrodes 
to allow the plates of one to be inserted between the plates of 
the other, in this manner allowing positive plates to alternate 
with negative ones, and thus providing the shortest path for 
the current through the electrolyte. 

Two types of commercial storage cells are in use: The 
lead-sulphuric-acid cell, sometimes called, simply, the lead cell, 
and the nickel-iron-alkaline cell, known as the nickel-iron, or' 
Edison, cell. The names are derived from the chemical 
natures of the electrodes and the electrolytes. 

62. Chemical Action of Lead and Nickel-Iron 
Cells. —In the lead-sulphuric-acid cell, the grids, both 
positive and negative, are of lead or of lead-antimony alloy. 
The active material of the positive plate when the cell is fully 
charged is lead peroxide, a chemical compound of lead and 
oxygen. The active material of the fully charged negative 
plate is metallic lead in a spongy, porous state. The elec¬ 
trolyte is a solution of sulphuric acid, formed by mixing 1 part 
of pure concentrated acid with 2.5 parts, by weight (4.5 parts 
by volume), of distilled water. The specific gravity of the 
electrolyte—that is, the ratio of the weight of a given volume 
to that of an equal volume of water—is about 1.2. 

The lead and the oxygen in lead peroxide are chemically 
combined into a substance from which neither can be separated 
except by a chemical process. The lead peroxide undergoes 
such a process during the discharge of the cell; half of the 
oxygen is transferred from the positive to the negative plate, 
producing on each plate lead monoxide, which is another 
chemical compound of lead and oxygen. At the same time, 
the sulphuric acid is decomposed into water and a gas called 
sulphur trioxide; this gas combines with the lead monoxide, 
forming lead sulphate on each plate. The active material on 
each plate of a fully discharged lead cell is therefore lead 
sulphate; and the electrolyte has become weakened because 
of the presence of additional water formed by the decom¬ 
position of some of the sulphuric acid. 


102 


OPERATION OF 


During charge, the reactions are reversed: the acid is 
restored to the electrolyte; the active material of the positive 
plate is oxidized to lead peroxide, and that of the negative 
plate is reduced to spongy lead. 

It will be noted that the specific gravity (strength) of the 
electrolyte decreases during discharge and increases during 
charge, thus furnishing an indication of the state of discharge 
of the cell, which state may be ascertained by means of a 
hydrometer. 

63. In the fully charged nickel-iron cell, the active 
material of the positive plate is nickel peroxide, and that of 
the negative plate is finely divided metallic iron. The elec¬ 
trolyte is a dilute solution of potassium hydroxide, or caustic 
potash. A small quantity of lithium hydroxide is added to 
the electrolyte to improve the capacity of the cell. 

During discharge, part of the oxygen of the nickel peroxide 
is dissociated and transferred to the negative plate, where it 
combines with the iron to form ferrous (iron) oxide; but the 
composition of the electrolyte remains unchanged. Unlike the 
electrolyte of the lead cell, the potassium hydroxide serves 
merely as a carrier of oxygen from one electrode to the other. 
When the cell is fully discharged, the active material of the 
positive plate is nickel oxide and that of the negative plate, 
ferrous oxide. 


CONSTRUCTION AND CHARACTERISTICS OF 
STORAGE CELLS 


CONSTRUCTION 

64. Lead Cell.— The component parts of the lead cell 
are the element, comprising the positive-plate group and the 
negative group, including connecting straps, or bus-bars, and 
the separators; the plate supports (in lead-lined tanks) ; the 
container, consisting of a glass or a rubber jar or a lead-lined 
wooden tank; the electrolyte; the cover; and the insulating cell 
support. 




DYNAMO-ELECTRIC MACHINERY, PART 2 103 


Fig. 27 shows a lead cell with 
a rubber-jar container, part of 
the cell being shown broken 
away in order to display the ar¬ 
rangement of the interior. The 
positive plate is at a ; b is a rub¬ 
ber separator; and c, a wooden 
separator. The plates rest on 
ribs d on the bottom of the jar. 

This type of cell is provided 
with a hard-rubber cover which 
has a hole for filling that can 
be plugged with a soft-rubber 
stopper e, having a small vent 
hole in the center. 

Cells in glass jars are sup¬ 
ported on shallow trays of 
wood or glass. The most satis¬ 
factory support for cells with lead-lined tanks is the oil insula- 




Fig. 28 Fig. 29 

tor, an annular trough, half filled with oil and covered with a 
lead cap. The glass trough rests on an earthenware pedestal, 
four of these insulators supporting one cell. 



























































104 


OPERATION OF 


05. Nickel-Iron Cell. —The plates of the nickel-iron 
cell are separated from each other by vertical strips of hard 
rubber, square in section, as shown in Fig. 28, which is a view 
of a cell from above. Sheets of hard rubber are inserted 
between the outside negative plates and the jar. The plates 
rest on hard-rubber bridges on the bottom of the jar. The 
container, Fig. 29, is a box made of nickel-plated sheet steel, 
corrugated to give added stiffness, the cover being welded on 
after the element is in place. The two terminal posts a and b 
pass through circular openings provided with rubber bushings. 
The central opening, used for filling the cell, is closed by a 
spring cap containing a valve c which allows the gases to 
escape, but excludes the external air. 


CHARACTERISTICS 

66. Lead Cells. —The capacity of any storage cell, 
expressed in ampere-hours, is the product of the rate of dis¬ 
charge in amperes by the number of hours the cell will main¬ 
tain that rate at full charge. With lead cells the ampere-hour 
capacity varies with the rate of discharge, being less at high 
rates than at low rates. The capacity of a standard stationary 
cell is based on the normal, or 8-hour, rate of discharge. 

The external voltage of a cell is that which can be mea¬ 
sured by connecting a voltmeter across the cell terminals; it 
is equal to the internal voltage minus the drop through the 
internal resistance. When the cell is delivering no current, the 
internal and external voltages are equal. 

The open-circuit voltage of the lead cell is from 2.05 to 2.08. 
During discharge the external voltage drops below the open- 
circuit voltage by an amount that varies with the rate and 
duration of the discharge. The final voltage at the end of 
discharge at the normal rate is 1.75. 

67. When a charging current is passed through a cell, 
the instantaneous rise of voltage above the open-circuit value 
is due to the internal resistance, and the subsequent gradual 
rise is due to polarization caused largely by the acid becoming 



DYNAMO-ELECTRIC MACHINERY, PART 2 105 


stronger in the pores of the plates. When the charge is near¬ 
ing completion, a further rapid rise in voltage is caused by the 
collection of bubbles of oxygen and hydrogen at the surfaces 
of the positive and negative plates, respectively—the result of 
the electrolyte being decomposed when practically all the active 
material has been fully charged. The final voltage at the end 
of a charge at normal rate, with the charging current still 
flowing, is from 2.6 to 2.8 for new cells. For older cells the 
voltage may not exceed 2.4 to 2.5. 

68. The specific gravity of the electrolyte decreases during 
discharge and increases to the initial value during charge. 
In stationary cells, the maximum value is about 1.210, which 
drops to 1.170 or 1.180 at the end of a complete discharge. 
When this range is once known the state of charge of a cell 
may be ascertained at any time by observing the specific grav¬ 
ity by means of a hydrometer. On movable cells, the 
maximum specific gravity is about 1.275. 

The foregoing data are based on a cell temperature of 
70° F. The capacity of a cell decreases with a decrease in 
temperature. The loss amounts to about .6 to 1 per cent, of 
the 70° capacity for each degree reduction in temperature. 

69. Nickel-Iron Cells. —The rated capacity of the 
nickel-iron cell is based on a 5-hour discharge rate. The 
actual capacity in ampere-hours, however, is but little affected 
by variation of the discharge rate, provided no limit is set to 
the final voltage. The open-circuit voltage is about 1.5 volts 
when fully charged, and the voltage at the end of discharge is 
rarely carried below 1. The internal resistance is approximately 
three times that of the lead cell of the same capacity and 
voltage and its efficiency is lower than that of the lead cell under 
similar circumstances. 

The principal advantages of the nickel-iron cell are: 
durability, mechanical ruggedness, and ability to withstand 
neglect and abuse without injury. It is best adapted for ser¬ 
vice at low discharge rates where cost of charging current is 
low, and where light weight is important. 


10 G 


OPERATION OF 


CHARGING STORAGE BATTERIES 

70. Methods of Controlling: Charge. —The charging 
current of a storage battery can be controlled by means of a 
rheostat, by varying the voltage of the source of the charging 
electromotive force, or by means of a booster. Only the first 
two will be briefly considered in this Section. 

71. Charging; Through Resistance. —A charging 
rheostat is connected as at r, Fig. 30, where the voltage of the 
charging source is greater than that required for the number 
of cells in series. In such cases, the voltage of the charging 




Fig. 30 Fig. 31 


source should be approximately equal to the final voltage of 
the battery at the end of the charge; the rheostat serves to 
reduce this voltage to that required at the beginning of the 
charge, and the resistance is gradually cut out as charging 
proceeds, so as to maintain the proper strength of the charging 
current. 

A few small cells can be conveniently charged from a light¬ 
ing circuit through lamp resistance. The current consumption 
of the lamps will then determine the charging current. Fig. 31 
shows a method of connecting a battery to charge from a 
110-volt circuit through five 110-volt, 16-candlepower, 

























DYNAMO-ELECTRIC MACHINERY, PART 2 107 


^-ampere lamps connected in parallel, the charging current 
being practically 5XJ = 2J amperes. The charging current 
passes through the switch a, the fuses b, the battery c, and the 
ammeter d, if one is used. The lamps may be connected in 
either lead to the battery. 

72. Charging by Variation of Charging Electron 
motive Force.— The small storage cells used in miners’ lamps 
may be charged by an arrangement similar to that shown in 
Fig. 30, except that the cells are arranged in parallel instead 
of in series. This is made necessary by the fact that the lamps 
are submitted for charging at irregular times, and that a series 
connection would require a constant readjustment of voltage. 
The usual arrangement is to provide two bus-bars from which 
the cells may be suspended for charging, the current at the 
required pressure being provided by any convenient means, 
such as a motor-generator. 

73. Storage batteries for mine locomotives may be charged 
from the lighting circuit, usually of 250 volts, or by means 
of a motor-generator set. The latter method seems to be 
preferable, as it involves less loss of energy. For example, 
supposing the battery to consist of 48 cells, and that the 
final charging voltage is 2.6, the voltage required for charging 
the battery will be 48X2.6 = 125 volts, nearly. A motor-gen¬ 
erator may be adjusted to deliver a current of this voltage; 
but, if the lighting circuit is to be employed, the pressure of 
the charging circuit will have to be reduced by 125 volts in 
the rheostat, showing that the. latter will consume one-half 
of the available energy. 

Complete motor-generator sets and automatic switchboards 
suitable for charging purposes are furnished by the manufac¬ 
turers of electric locomotives. 






























